The matrix looks at the use of transparency [Transparency] with different settings of part color and part material in relation to different settings of face color and face material.
Explanations of individual combinations
part color: Use of transparency from color and texture [Color and texture] → Color. Here, the transparency is set by the fourth value.
part material (mix=false): Edit [Edit material for 3D preview] use of transparency from material for 3D preview [Edit material for 3D preview].
part material (mix=true): Edit [Edit material for 3D preview] use of transparency from material for 3D preview [Edit material for 3D preview].
part color + part material (mix=true): Edit [Edit material for 3D preview] use of transparency from material for 3D preview [Edit material for 3D preview].
There is no transparency for the SYS_FaceAttributeColor (face color) face attribute. Therefore, the transparency setting of the part always wins here, either from Color and Texture [Color and texture] → Edit [Edit material for 3D preview] color [Color] or from Material for 3D preview [Edit material for 3D preview].
face material | face color + face material | face color + face material mix