The ALLOCATE parameter controls the storage allocated for members when they are added to the file (CRTPF, CHGPF, CRTSRCPF, and CHGSRCPF commands).
The storage allocated is large enough to contain the initial record count for a member. If you do not allocate storage when the members are added, the system will automatically extend the storage allocation as needed. You can use the ALLOCATE parameter only if you specified a maximum size on the SIZE parameter. If SIZE(*NOMAX) is specified, then ALLOCATE(*YES) cannot be specified.