If true, then always keep any files that arise during the use of runExternalM2.
If false, then always delete these files.
If null (the default), then keep the files only if the job was a failure, meaning that an understandable result was not able to be read from the temporary file storing the results of the computation. If the job was unsuccessful (e.g., exit with nonzero error code, or the process was killed by the operating system) then the files will be saved to aid investigation.
All of the files involved were created using temporaryFileName in the parent Macaulay2 process, and hence will always be deleted upon termination of the parent, no matter this setting.