Hello Luis
The terms import and export have to do with the specific commands placed within the VRF used for the particular customer connected to that PE router. The route-target export command is used to export routes from the CE into the PE router and the MPLS domain. The route-target import command is used to import routes into the CE from the PE.
Now because often the same RT is used for both import and export, this is hard to see. Take a look at the following lesson:
In it, different RTs are used for import and export. You will notice that PE1 uses the 1:1 route target to export prefixes from CE1, and when PE2 receives these, they are indeed appended to RT 1:1 (and not to 2:2 which is the RT for the import command).
I hope this has been helpful!
Laz