I'm trying to develop some custom spells based off of existing effects, and I'm not understanding what is populating the contents of the drop down box when I create a new effect in a spell. In order to test my idea I was trying to add a soul trap effect to a firebolt spell. When I look at the effects used by the current soul trap spell, it is using the effect SoulTrapFFActor. But when I add an effect to Firebolt, this effect isn't available from the effect drop down. I do find EncSoulTrapAimedArea, which appears to be the weapon enchantment version.

As I'm browsing through the list of all enchantments, I see that SoulTrapFFActor is a script, so I'm thinking that must be part of the problem. But trying to add an effect to the Soultrap spell instead of Firebolt, I see a completely different list.

Anyway, testing out the modified Firebolt spell, it will indeed trap a soul. I get the message that a soul has been captured and can verify in my inventory that my filled gem count is higher, but there is no visual effect. I assume that is because nothing is calling the Papyrus script "magicsoultrapfxscript" that is called by the SoulTrapFFActor script.

So what is governing the content of the effect drop down? I'm not getting it.

If there is a tutorial out there that covers spells and spell effects, I'd be happy to take the time to watch it, but I didn't see one in the sticky above.

You can only add magic effects with the same target than the spell, so for example if the spell with Firebolt is aimed, you have to add or create an aimed version of Soultrap effect to be able to add it to the same spell.