I assume you must already have the
68HC12 instruction set reference; the big table that lists the address modes, machine coding, number of bytes, and affected flags for each instruction. And you already decoded 0x2D as
BLT (branch if less than).
The address mode REL means that the next byte is a relative offset from the program counter. Since the offset is 2's complement, it can be positive (skipping ahead) or negative (looping back).
The tricky part is you have to be careful about whether it's relative to the program counter address of the BLT instruction itself, or the address of the next instruction. (This can be different for different types of microcontrollers.) This is explained in the