The error IDADERRP51E - INTERR: OUTPUT LIB is full will be displayed when the ADROUT file has reached the value set for Percent full occupancy warning in the PSS site options. This message is actually an error and not just a warning as it stops writing to the ADROUT file.
The term Warning may appear a bit misleading, as a new output can't be opened once the percentage full is reached. However, the application that is writing the output that takes it beyond the threshhold is not affected, and that output is allowed to complete. So in that sense, the percentage is not a hard limit, and "warning" is as good a term as any.
The percentage should be set so that all outputs will be able to complete, if that boundary is crossed. The worst case scenario is not if a very large report is begun when the library is one block short of the limit, because that will bring down the barrier to new outputs almost immediately. Multiple reports being written simultaneously is more of a concern, as you want all of them to complete before the library really does run out of space. So available free space needs to be report size times number of simultaneously open outputs.
Applications will fail when they try to open a new output, and of course, a PRINT command will do nothing. At that point, there's nothing to back out, and it's just a case of "try again later". Typically, unwanted outputs will be deleted to free up space, and normal service is resumed. The administrator can temporarily set the threshold higher, to allow a small print to be created, but that's not recommended as a general practice.