EPL

EPL is a container used for storing particle data. In Persona 3 and Persona 4, EPLs are used for Cutins and often contain RMD models, their extention in the PS2 games can be EPL or CPK and CIN which despite the name have no relation to P5/Criware's CPK as internaly they are still .EPL. In Persona 5, EPLs are sometimes embedded in GMD models or even animations. The all-out-attack finisher art is stored inside EPL files as well.

Tools
Currently there are no programs that are able to modify particle data. However, you can extract and replace Persona 5 GMDs with EPL Injector on top of that the AtlusGfdEditor and P5 GMD Maxscript can ignore EPL data, allowing you to import, export and modify GMD files even if they have embedded EPLs.

For Persona 3 and 4 EPL files there is a set of commandline tools that allow for extracting and importing back of all embeded files.

Python Scripts
ThatTrueStruggle's EPL Extractor and EPL importer can be used to modify RMDs from P3 and P4 EPLs. Using them with Python 2.7 and the command prompt, you need to open the command window, type py -2 epl_extract.py, and paste the filepath of your EPL. There's a python 3 version of epl_extract.py here's the link EPL Extractor python 3 build