[WhiskyTangoFawks]

xEdit defines EPFB, EPF2 and EPF3 as function parameters any perk effect can carry, and Mutagen reads them onto every effect. Its writers wrote EPFB only for an entry point and EPF2 and EPF3 only for an activate choice, so a rewrite dropped them from any other effect. Separately, a Fallout 4 Add Activate Choice entry point with no EPF3 (e.g. FTS_FastTravelSettlement.esp) threw on read, though APerkEffect.Flags is already nullable, and its writer threw on a null Flags.

• Fallout 4 and Starfield write each of EPFB, EPF2 and EPF3 for any effect that holds it, in xEdit's order. An activate choice without EPF3 reads as null Flags, from a full import and from an overlay, and null Flags writes no EPF3.
• Skyrim writes EPF2 and EPF3 for any effect that holds them, in xEdit's order. APerkEffect.Flags is now nullable, as in Fallout 4 and Starfield, and holds a value exactly when EPF3 is present.
• The activate choice's own ButtonLabel and Flags initialisers go; the code after the switch already assigns both for every effect.

Breaking: Skyrim's IAPerkEffectGetter.Flags is IPerkScriptFlagGetter?, and a new Skyrim effect has no Flags.

Behaviour change for code-built records: an effect of any kind that sets PerkEntryID, ButtonLabel or Flags now writes them, where before they were dropped. An activate choice with null Flags writes no EPF3, where before Fallout 4 and Starfield threw and Skyrim wrote an all-zero EPF3.

Still read and discarded, unchanged: EPFT and EPFD on a quest or ability effect. Fallout 4's masters and a 500-plugin Fallout 4 load order carry neither.

PerkFunctionParametersTests covers, for Fallout 4 and Starfield, an ability carrying the parameters and an activate choice with and without EPF3, by full import, overlay, and write then reread through the other path. For Skyrim, whose full import and overlay share one effect parser, it covers an ability with a button label, and an ability and an activate choice with and without EPF3, by write then reread.

[Noggog]

Making it nullable, as in Fallout 4 and Starfield, would close that, at the cost of an API change. Happy to do that here or separately if you'd like.

These types of API changes are acceptable within reason. Throw em in. In my view they're in a separate category than API changes to say, ILinkCache. Record definitions will always be evolving over time, realistically. Nature of this specific beast

[WhiskyTangoFawks]

Done: Skyrim's Flags is now nullable, so any effect keeps its EPF3, and an activate choice without one no longer gains an all-zero EPF3 on write. PerkFunctionParametersTests covers an ability and an activate choice with and without it for Skyrim.