Only printer files opened with SPOOL(*YES) are allowed in a job that is capable of multithread operations.
Unspooled opens of printer files fail with a CPF4380 (Open attributes not valid in a multithreaded process) escape message sent to the function that requested the open.
Most SNA character string (SCS), Intelligent Printer Data Stream™ (IPDS™), Advanced Function Printing™ data stream (AFPDS), LINE, and USERASCII write operations are threadsafe. Write operations that are not threadsafe include those that use SCS graphics, those that the System/36™ Environment uses, those generated by the User Interface Manager (UIM), and those that use System/36 PRPQ subroutines. Page and line counts might be inaccurate because those counters cannot be atomically updated.