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em/blocksize_KB: Segment Size of the
Extended Memory 
This value specifies the size in kilobytes of the segments that are assigned to the SAP Extended Memory.
If a user context requests extended memory, a segment of this size is allocated from the extended memory pool.
The default value is platform-specific and is determined dynamically. The default value is specified in transaction RZ11. This value should not normally be changed.

Only SAP is allowed to change this parameter.