If you asked me (which you haven't) I'd say that ".esl" and "espfe" plugins are the best thing to come out of Fallout 4. Here I'll explain what an esl is, what it isn't and how it can make your life past the 255 plugin limit an absolute heaven when compared to F3/NV.

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The ".esl"

".esl" plugins otherwise known as "light masters" are a new plugin type added with SE and the Creation Club, you may notice that most CC mods use this plugin format, it was probably created because Bethesda wanted to make a plugin type that wouldn't count towards your 255 plugin limit, and didn't want Creation Club mods to lock players out of achievements when downloaded.

Here's a comprehensive list on what ".esl" files can and can't do (from creationkit.com):

• • .esl files cannot be edited in the Creation Kit. Active development should be done in an normal .esp file. (Edit as a .esp, export as .esl)

  • .esl files allow more plugins to be loaded than the previous 8-bit plugin ID limit (255), by using more of the Form ID for their load order.

  • .esl files have a budget of no more than 2,048 forms, with Form IDs 0x800 to 0xFFF available for use.

    • A absolute maximum of 4096 .esl files can be loaded by the engine at once, but in practice this is difficult to achieve.

    • 331 plugins can be loaded that each contain 2048 ALCH records, but 970+ plugins can be loaded that each contain one CELL record and 2047 REFR records.

    • There may also be a file handle limit, and a reference limit that prevent 4096 .esl files from being loaded.

  • .esl files are merged into plugin slot 0xFE at runtime. Their runtime Form ID is 0xFE[000](Hex Load Order)[000](Original Form ID).

    • Form 0xFA0 from the 10th loaded .esl file would be 0xFE00AFA0 at runtime. From the 200th .esl file, it would be 0xFE0C8FA0.

• To convert a normal plugin (.esp) to a light (.esl) master, load your .esp file, and under File, select Convert Active File to Light Master.

• It is good practice to run Compact Active File Form IDs to optimize your Form IDs within the 2,048 form budget before converting.

  • Compacting Form IDs does also change Form IDs in the .esp file, so be sure do this before converting to .esl, and be sure to save the .esp file afterwards.

  • Do not compact Form IDs on an .esp file you have already released and are updating. Because Form IDs change, you essentially create a new plugin when compacting, and this new plugin will not be compatible with saves using the old version.

• Updating .esl files has more restrictions than updating .esp files, but not many.

  • Because of the 2,048 Form ID limit, it is important to be careful when deleting forms not to reuse Form IDs.

    • A saved weapon form could be deleted in your plugin, then have an armor form take the FormID when compacted.

  • Again, be sure to save compacted .esp files, because otherwise compacting again with new forms might give different Form IDs for forms already in a game save.

  • As long as development is done on compacted .esp files that match released .esl files, forms can be added under the 2,048 form limit, and existing forms can be edited without issue.

• .esl Files released by Bethesda Game Studios (Creation Club releases) are flagged as master files and always load immediately after the official Bethesda masters, in the order specified by the game's .ccc file. These .esl files cannot be deactivated.

• .esl Files released by modders are flagged the same as .esp files, but these also load with the master files, in the order specified in plugins.txt. These .esl files will need to be activated before the game will load them.

• .esl Files created by modders cannot use standard .esp files as masters or this will cause the .esp file to also load before the .esl in the master files section. This is new behavior for the game engine as of Skyrim Special Edition v1.5.3.

  • Any dependencies of the normal plugin a light master depends on are not loaded early.

  • Creating light masters that are dependent on normal plugins is highly discouraged.

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The biggest takeaway from that list is the fact that ".esl" plugins, while small and limited, don't count towards your 255 plugin limit, instead they have their own limit of 4096 plugins instead, This is great news for those people that would constantly have to make Merge patches to get around the plugin limit, instead we can just turn smaller mods into .esl plugins and live happily past the limit! There are even more good things we can now do with this new plugin form, which is make ".esp" plugins with a ".esl" form, now called espfe plugins.

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The "espfe"

The "espfe" file is a ".esp" labeled as an ".esl". It is named espfe because its a .esp plugin that loads on the "fe" space, just like ".esl" files, which means it benefits from bypassing the plugin limit, while also loading on a normal load order like ".esp" files can, meaning we can turn compatibility patches and other smaller mods into espfe. Instead of writing a huge wall of text on what these magical plugins can and can't do, I'll direct you to read Qwinn's incredible article on the subject detailing all that is and isn't about these files. >>>Read here!<<< and >>>read here<<< for a tutorial on how to turn ".esp" plugins into espfe's!

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The downside

The biggest downside is the fact that ".esl" and espfe mods cannot be merged, not that you would need to since they have a limit of 4096 plugins, but its a limitation that you should keep in mind.

And, while probably only a temporary issue, espfe plugins are a relatively new thing and not all tools have been updated to work properly with them such as Mator Smash or Merge Plugins. Wrye Bash's development build (available on their Discord) has a working version that identifies these plugins.