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. In Persona 5, EPLs are sometimes embedded in GMD models. 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 their 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.
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 just need to specify a path to the EPL you are working with.