Program counter format LC-2
The notation is as follows: FROM[left bit : right bit], so in your example PC[15:9] means take bitfield (number) from PC from fifteenth bit to ninth bit, inclusive, so 15 - 9 + 1 = 7 bits.
The same applies for IR[8:0] meaning take bitfield from IR from eight to zeroth bit inclusive.
For example PC[14:14] would be exactly one bit long and in such case by convention one would write PC[14].