The PR for subaddresses and MRL-0006 say that the value m is calculated as:
m = H_s(a, i) or H_s(a || i)
I have found another question attempted to find the answer to my same question, however I am not sure if it is up to date and also is not completely specific. (How is m calculated when generating subaddresses)
Is this prefix going to be included in the actually v0.12 release? I cannot find any related code in the v0.12 branch on Github. Additionally, is the i value varint encoded? In the code linked at the above question, it looks like it simply would append the two uint32_t values.