Transports in SAP are done using the tp command on the OS level, this post tries to explain the different modes available to further customize your import.
tp import transportrequest clientxxx U1
tp import transportrequest clientxxx U126
tp import transportrequest clientxxx U148
The u stands for unconditional force ( how hard you want to force the request into the system – do you want to overwrite without warnings, or softly try to load the objects).
0: Called a overtaker; change request can be imported from buffer without deleting it and then uncoditional mode 1 is used to allow another import in the correct location.
1: If U1 is used with the export then it ignores the correct status of the command file; and if it is used with import then it lets the user import the same change request again.
2: When used with tp export, it dictates the program to not to expand the selection with TRDIR brackets. If used in tp import phase, it overwrites the originals.
3: When used with tp import, it overwrites the system-dependant objects.
5: During the import to the consolidation system it permits the source systems other than the integration system.
6: When used in import phase, it helps to overwrite objects in unconfirmed repairs.
8: During import phase it ignores the limitations caused by the table classification.
9: During import it ignores that the system is locked for this kind of transport.