it's referring to the word "batch" it's showing this way (only example) the 1st 10 people coming to APU from Indonesia called Angkatan 1 the 2nd one 15 people coming, and called Angkatan 2 in 1 year there are 2 times recruitment ( Spring and Fall) so Angkatan 16 means 8 years have passed since 1st one from Indonesia coming to APU