For outbound requests, up to 32 bytes are allowed, although it is expected that requests that are sent to another TXSeries for Multiplatforms region will be four bytes or less. The APPC architecture allows up to 64 bytes, but leaves each product free to set its own maximum. TXSeries for Multiplatforms complies by allowing 32 bytes, but this need concern you only if your TXSeries for Multiplatforms region is connected to a partner system that demands longer transaction identifiers.
For inbound requests, however, the transaction identifiers must be four bytes or less. TXSeries for Multiplatforms does not support inbound requests for transaction identifiers that are longer than four characters.
The slight difference between IBM mainframe-based CICS and TXSeries for Multiplatforms with the use of transaction identifiers, in that IBM mainframe-based CICS truncates down to four bytes while TXSeries for Multiplatforms does not.
Refer to the description of the PROCNAME attribute in CONNECT PROCESS.