If REGION is coded as 0K or 0M, how much storage would be assigned to the job
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default storage; 255M
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All available storage
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will result in JCL error
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none
In JCL, REGION=0K or REGION=0M specifies that no storage limit should be imposed on the job. This means the job can access all available storage in the system address space it needs, up to the installation-defined limit. It does not mean a specific default value like 255M, and it is valid JCL syntax.
On z/OS, coding REGION=0K or REGION=0M is a special case that tells the system to give the job step as much storage as is available (up to the installation's defined maximum), rather than truly zero storage. It's a common misconception to read 0 literally as "no memory," but the region parameter treats it as an explicit request for the largest possible allocation.