Terminologies
Meshβ
Componentβ
Component is part of the Mesh that has no connection with other parts of same Mesh
Mergeβ
Merge operation is adding several independent Meshes into one Mesh making components from them (addMesh() and addMeshPart() functions)
Boolean Unionβ
Boolean Union operation stitches intersecting Meshes making new Mesh that is composition of input meshes (it cannot work with some bad defined cases like non closed intersection contours or self-intersections in this zone) (also if meshes does not intersect this operation works a lot like merge but with respect to meshes orientations)