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 extension in the PS2 games can be EPL or CPK and CIN which despite the name have no relation to P5/Criware's CPK as internally 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.
Currently there are no programs that are able to modify particle data. However, you can extract and replace Persona 5 GMDs with SecreC.'s EPL Injector Tool. 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 command line tools that allow for extracting and importing back of all embedded files.
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 file path of your EPL. There's a python 3 version of epl_extract.py here's the link EPL Extractor python 3 build