Yea, I must admit I was a little afraid this could become confusing. The methods to play sound effects and music effects are called play_se and play_me. Why a different name? Because the first version of the plugin was not coded by me and I actually never decided which one to stick with. I'll most certainly change it for constistency as soon as I can, that's on me.
About the other question: MFX or ME or Music Effect usually refers to something like a sound effect but more... Musical? Like a jingle or a short fanfare. I guess you could technically consider them still sound effects, but back when I liked to mess around in RPG Maker they were separated, so I thought it made sense to also do it myself.
Just to clarify: you can do whatever you want. The methods play_bgm, play_bgs, play_se and play_me behind the scene call the exact same method: play. The only difference the module makes between a sound type and another is the default properties (volume and pitch) and the AudioBus. So you could technically use the same bus and properties and just use play_bgm for every sound you want to play and it shouldn't make any difference whatsoever.
If you're still confused about the sound type I used, let me explain. Basically I used the same types I found back in the days in RPG Maker, as I said, as I thought it covered up everything needed:
BGM stands for Background Music. It refers to, well, background music, like a soundtrack you want to play during a scene or a level.
BGS stands for Background Sound. It refers to any sound you want to play in background, like rain or a crowd talking, or the wind blowing. Think about a background music, but replace it with a sound.
SFX and SE stand for Sound Effect. They refer to, well, sound effects. Like a gunshot, or a laugh, or a short applause. Everything that's a sound but it's not supposed to just loop in background, but to end soon.
MFX and ME stand for Music Effect. As I already explained, think about a sound effect and replace the sound with a music. A victory fanfare, or a saved game jingle. A music that's not supposed to go on for long.
Hope I clarified everything, I'm sorry if the plugin wasn't really clear on some part! But I still hope it's doing its job and being useful for your projects! I will fix that inconsistency in the semantic as soon as I can (unfortunately I can't use my pc for now, so it'll have to wait).