Allocation type column in DBA_TABLESPACES

When I browsed through the reference manual, I noticed that the allocation type can have the following values: SYSTEM,UNIFORM and USERSYSTEM  is "EXTENT MANAGEMENT LOCAL AUTOALLOCATE". UNIFORM is, of course, EXTENT MANAGEMENT LOCAL UNIFORM SIZE <size>. What the heck is "USER"? I'd like a pointer to the appropriate document, if possible. TIA

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Mladen Gogala