Every undo log block consists of a Block Header (MVSLogBlockHeader type), followed by log records. Every log record includes a standard General Log Header (GenLogRecord type), followed by record data.
Record data consists of a fixed part (the prefix), and an optional variable part (the caller data).
Undo Start and Undo End records (record data) consist of only a fixed prefix. Undo Read-for-Update/Erase and Undo Write-for-Add records (the record data) contain caller data as well.