https://en.m.wikipedia.org/wiki/Functional_requirement - In software engineering and systems engineering, a functional requirement defines a function of a system or its component, where a function is described as a summary (or specification or statement) of behavior between inputs and outputs.
Reverso

I think it's probably best to define the base term rather than just its non alternative.

@@ -5 +5 @@
-<keb>非機能要件</keb>
+<keb>機能要件</keb>
@@ -8 +8 @@
-<reb>ひきのうようけん</reb>
+<reb>きのうようけん</reb>
@@ -12 +12,3 @@
-<gloss>non-functional requirement</gloss>
+<field>&comp;</field>
+<gloss>functional requirement (of a system or component)</gloss>
+<gloss>capability requirement</gloss>

https://ja.wikipedia.org/wiki/非機能要件