how can we use save management in assembly files?
"Save all" -- saves every part or product for each modification very easly.
"Save management" like name is management. Save component parts, assemblies, and drawings (several documents) independently at once in management window. And also lets you save all your modified documents under a new name and a new location.
To expand on what Ozgur said:
Save Management is very useful to maintain links between files, especially when files are renamed or relocated into other folders. Examples of links are: View Links between a drawing and a part, Instance Links between an assembly and a part, Import Links between two parts, etc.