Tape block size considerations
File Manager can process input and output tape blocks that are larger than 64KB. The theoretical limit for a tape block is 10MB.
If you use tape blocks that are larger than 64KB (32KB for labeled tape processing), be aware that other software may not be able to process such large tape blocks. The hardware that you are using may also limit the block size.
File Manager supports LBI (Large Block Interface) for tape. Note that data sets (which require LBI) can not be allocated while using the ISPF interface. However, existing data set (or data sets) pre-allocated using JCL (in batch) may be processed for both input and output.