What's the difference between holding social security files of flexible employees and trusteeship?

The difference between the two is that you hold it yourself and keep it yourself. Trusteeship is a means for flexible employees to keep files.

So one is the form of employment, and the other is the means of keeping files, which is very different.

Unit custody is to charge some custody fees. Because personal files are sealed, in order to avoid being lost or tampered with, neither I nor others can keep files.