Oracle Database Discussions

Hi,
As we know
"User space quota can also force the space pressure situation even though there is free space in tbs. In this situation oracle will purge few data from Recycle bin(Logical Space) "

Now my doubt is:
When we create a user in oracle we allocate some space for this user.Is this space is static or dynamic.Means if there is a space in tbs then why does such space pressure happen????