P1 u/s or ICUS has been around a long time. As mentioned in mad_jocks post it is used for low time pilots in multi crew ops so as to get enough hours for an ATPL.
I have P1 u/s hours from 40 years ago in the UK flying DC3s with 400hrs total. In those days the captain gave you the sector and if you flew it with little input, then he would sign your log book declaring it as a P1 u/s sector, half of those hours went towards that required for the ATPL.
BTW mad_jock I was also flying out of ABZ!