Always focus on the Icons that are present everywhere in the software. For an example on the ribbon and on the dialogue boxes of tools etc. They are self-explanatory and give you information about how a tool will function. So watch every icon very carefully.
For an example watch the dialogue box of the Mate Constraint too. To activate this tool just go to the Assembly tab>Position panel and click Constrain tool to activate it. Now the Place Constraint dialogue box will open. The mate constraint is active and default solution type selected is mate. Focus on the solution section and see what the icon shows. Watch the image below.
This Icon clearly states that mate constraint will work on entities that are in opposition to one another.
So when mating two components in an assembly always remember this. And where the red arrows show the opposite direction after selecting the planes, then the mate solution will work. Many a time the users think which mate solution, he should apply. So when red arrow shows the opposite direction then select mate and when the red arrows are showing like the Flush solution icon then select the flush solution. This is very helpful while working with a large assembly which consists of many components. To better understand it see the video below:-
