it depends on the + vector of the 2 faces selected.
I believe that
Mate = Surf A with +Vector -to- Surf B with +Vector
Mate = Surf A with -Vector -to- Surf B with -Vector
Plainar align = Surf A with +Vector -to- Surf B with -Vector
Plainar align = Surf A with -Vector -to- Surf B with +Vector
Assuming this is correct
Mates with a positive offset value result in a "gap" between the faces
planar align differs in that a positive offset value results in the B face being "inboard" (A-) of the A face, and a negative offset value moves the B face "outboard" (A+) of the A Face
maybe some else has a better way to describe this or pictorially display it?
I just toggle and invert as needed...
This would be more initiative if the origin arrows were in the direction of the planes "+ normal direction" X and Z are, but Y is negative relative to the arrow. The real trick is getting the offset to be + so that the math is easier in variables.
never really thought of this before, but you are correct...
I have just always known to establish origin via the X axis marker on the plane and thus normal is coming out of the screen at me...
It took me a many years of frustration before I finally realised how this works, and its not easy to explain in words.
The distance is measured along the surface mormal of the part being positioned, starting from the surface.
So, assuming your surface is a face of a solid, positive is away from the solid, negative is 'into' the solid.
My guess as to why things are this way is the differance between drafting conventions in the USA vs. the rest of the world.
starts in lower left corner
3rd angle projection
right hand rule
REST OF THE WORLD
starts in center
1st angle projection
left hand rule
The reason this is important, w/ left hand rule, the XYZ axis are set up as theya re in SE. This (the Y axis) is backwards for people in the USA.
since with the alignment relation both normals point in the same direction, there are 2 solutions to solve a distance.
In Solid Edge the alignment offset is defined relative to the normal of the first face.
So if it is positive, then the second face is "in front" of the first.
If it is negative, then the second face is "behind" of the first.
When you look at this scenario from the second face, then in front or behind changes.
You can compare this to a car race: the first car is in front of the second (seen from the second car= positive offset), but seen from the first car, the second is behind (= negative offset)
The problem is, when you edit the relation, it is not obvious which planes is the first. But when you define a new relation, then it is clear which is the first and the second face.
There is no difference, if the other part is fixed or has other restrictions.
This has nothing to do with right/left hand rule, any coordinate system or the projection method (only in 2D)
Hope this was not too complicated ;-)