The difference is, among other things, the CAS latency; this is mentioned in the manufacturer's number
For G. Skill:
for F4-3000C16D-16GTZR with CAS latency 16 ; tRCD 18 ; tRAS 18 ; tRP 38
means c16 = CAS latency 16
Accordingly :
for F4-3000C15D-16GTZR with CAS latency 15 ; tRCD 16 ; tRAS 16 ; tRP 35
means c15 = CAS latency 15
CAS latency : see Wikipedia
https://de.wikipedia.org/wiki...
more detailed English version :
https://en.wikipedia.org/wiki...