"We assemble these parts and make wire harness"
If all you are doing is putting the component parts together and are not modifying them from their published catalog data you are dealing with an assembly FAI.
Now say you are modifying a purchased component (catalog item) you are going to perform a FAI on the modification you performed you are now dealing with a detail FAI.
For the wire harnes example say you purchase wire and conectors and such. Now from the spool of wire you cut off a 1 ft length +/- .35 inch - per print- and strip the insulation off of the ends .25 inchs +/- .010 and then crimp the pin on and ensure that it has the required pull of strength.
This prepared section of wire that will go into your harness is a detailed part since there is variable data to check for compliamnce to design.
When you put 5 of these into the conector you now have an assembly since there is no variable data to check other than you have the correct componets.
Yes this is a bit simplified since there may be specific requirements for the fabrication of the conector such as typicaly a color code or wire size in a specific conector location but you get the idea I hope.