Its that note 5 which is up for debate.
Provided that the method of supervision is acceptable to the auth..... etc
Its not defined what this is unfortunatly.
The bit that some say you can't be classed as PICUS flying as a FO.
"when all the functions and dutys are carried out such that......"
Which the argument goes that to be doing that there must be complete role reversal including tech log etc etc. Which most SOP's preclude you from doing. And i have heard of some captains refusing to sign because of this reason.
I suspect for the ex-instructors they really can't be bothered as the PIC hours are more than required without counting any PICUS hours.
Letter from the company stating that your log book entrys are correct seems to be the easy option. And also keeps you clear of the different opinions on the subject. I haven't heard of anyone being knocked back with a letter (or with out to be honest).
MJ