tail() returns the last 6 items of a subsettable object. When using aggregate(), the parameters to the FUN argument are passed immediately after the function using a comma; here 1 refers to n = 1, which tells tail() to only return the last item.
aggregate(a[, c('NUM', 'VAL')], list(a$ID), tail, 1)
# Group.1 NUM VAL
# 1 A 2 0
# 2 B 3 0
# 3 C 9 0
