When generating request units from a message sent with OPTCD=LMPEO, VTAM® gives no consideration to the contents of the data. In general, the points at which VTAM splits the data, the RU boundaries, can occur anywhere in the data. VTAM ensures only that no RU is larger than the applicable maximum RU size, and that the first generated RU is equal to the maximum RU size, if there is enough FM data to make this possible.

Because VTAM does not consider data content when splitting a message into RUs, LMPEO should not be used where RU boundaries imposed by LMPEO are significant relative to the data. Use of the buffer-list option, in combination with LMPEO, allows the application program to specify RU boundaries and, therefore, to minimize any adverse effects of RU boundary selection by LMPEO. Refer to The buffer-list (BUFFLST) option for information about how the buffer-list option is used with LMPEO.

Some cases in which data-stream architectures or device implementations place restrictions on the use of LMPEO are as follows:

1. For LU type 1, if card data streams are to be used, the data stream profile (DSP) in the BIND must specify that cards can span RUs; otherwise, LMPEO might place an RU boundary in the middle of a card image. If an LU type 1 implementation does not support this DSP, do not use LMPEO.
2. FM headers cannot span RUs for LU type 1. For LU type 1, all FM headers are contiguous and begin with the first byte of the chain. If the maximum RU size is equal to or greater than the largest concatenated set of FM headers sent by the application program to the LU, there is no problem with using LMPEO because the first RU generated for LMPEO data splitting is always equal to the maximum RU size. However, if the FM headers are larger, the application program can define RU boundaries for the part of the chain that has FM headers through use of the buffer-list option and LMPEO control flags. Alternatively, the application program can use multiple SENDs. It can send the FM header RUs with OPTCD=NLMPEO and send the rest of the chain using OPTCD=LMPEO.
3. For LU type 1, the string control byte (SCB) must be at the front of each RU. This is not guaranteed by VTAM when using LMPEO. Therefore, compression cannot be used with LMPEO unless the application program uses the buffer-list option and LMPEO control flags to define its own RU boundaries.