i made a performance with few parts from a library . if i erase the library, is still the performance multi part
contain the the edited part from library ?

When you make Performance, you assemble and Store it in the USER area of 640 Performances. You don't mention whether or not it uses custom Waveforms (if it does you must IMPORT the data from the Library). The act of Importing earmarks that data so you can create a new backup file that includes the custom Waveforms/Samples. Once you have your new backup file, then you can safely DELETE the installed Library.

if i want to store only a part of a performance multipart to a new performance, is there a quick way?

Again, the source of the Part is critical. Please see the recent article on "Using Montage Category Search". It explains how you can go and get any single Part, 1-16, from any Performance.

Say you recall the "INIT Normal AWM2" as a starting point (but the starting point could be the new Performance you are building) and you want to recall the very cool but wacky "Seafloor Kit" as found in PART 3 the "Marine Life" Performance, you can use the "Part Search" function to go and just get that Part (Source = Part 3) from the Performance.

When it is said that each Part of a Montage Performance is the equivalent of a Motif XF Voice, this means that individual Parts can be easily recalled from within Multi Part Performances. I like the Seafloor Kit because it shows how diverse a drum kit can be (vocoder samples, sound effects, noises, etc.)

Thanks
So if part are only fm. It is optional to save in user memory the original source of this part from library. I will be able to access this part even if the library no longer exist .

Correct. FM-X sounds are tiny in comparison to the data sizes involved with Samples.