p1,m1

These variables specify the character field in the input record for the substring test.

p1 specifies the first byte of the character input field for the substring test, relative to the beginning of the input record.¹ The first data byte of a fixed-length record (FLR) has relative position 1. The first data byte of a variable-length (VLR) record has relative position 5 (because the first 4 bytes contain the record descriptor word). All fields to be tested must start on a byte boundary and must not extend beyond byte 32752.
m1 specifies the length of the field to be tested. The length can be 1 to 32752 bytes.

¹ If your E15 user exit routine formats the record, p1 must refer to the record as reformatted by the exit.