You have to keep in mind that a persistent system is a Live system on which there is an overlay that is writable.
This means that core files are in read-only mode while other files are in read/write mode.
What you cannot do on a persistent system:
- updates core files (kernel, etc...) = no full system updates
- install drivers
What you can do on this kind of system :
- install some software
- do some software updates
- create some files/folders
- install Linux (from you USB key) on your PC's hard drive