Difference between form union and form assembly

I performed the "compose" operation on my geometry deleting some parts which I did not want, but then when "from union" those deleted parts showed up again in the final geometry.
Only when I do "form assembly" the unwanted parts don't show in the final geometry but then the software is not able to converge and giving errors.

I tried reading the documentation from the comsol multiphysics reference guide and here is what says there:

"If you form an assembly, select the Create imprints check box to get imprints of the
parts of the assembly that touch each other. Select the Create pairs check box to
generate pairs corresponding to the parts of the assembly"

"From the Pair type list, select Identity pair (the default) to generate identity pairs or select
Contact pair to generate contact pairs"

It does not explain much about what is the meaning of generating pairs or imprints.

Can anyone please help

Thanks

AG