A library or a file must exist in a contiguous organization on one fixed disk (however, a library may contain one "extent" of roughly 50 blocks which must be reorganized, and it cannot be extended if allocated to other users). A file may be organized with an EXTEND value or it may be allocated with FILE OCL to automatically extend. All record adds/updates/deletes wait while the file is being extended. It is good sense policy to create extend values large enough to minimize the frequency of extends. Libraries could have "extents" that were not contiguous. At times, when compiling a program, an extent would be created and by doing a "CONDENSE", it was removed if there was enough room in the main allocation for it. Otherwise one did an ALOCLIBR to reallocate the library to a bigger size.