i would like to know if someone can be freelancer for some works and customer for others.?..i mean not on the same project of course .
What is the policy of upwork about that ? maybe it is possibel with differents accounts or the same ? i really dont know...
Yes, you can be both, please check this page: https://support.upwork.com/entries/23074437
I am not both (I'm only a freelancer) and can't get into more details but I believe you need to have 2 different bank accounts. One will be for your freelancer revenue and the other for the client expenses.
However, I'm not 100% sure about this last statement, perhaps someone else could confirm this?
Olivier asked, "What is the policy of upwork about that ? maybe it is possibel with differents accounts or the same ? i really dont know..."
If you are going to be a client and a freelancer both, it MUST be with same account, so be very careful if you do this because if you are suspended as a client, you will also be suspended as a freelancer, and vice versa.