Reliability means success rate of transmitting packages.
URLLC is sending small packets like 32 bytes with a success rate of over 99.999% in less than 1msec delay.
This is possible only with a high SCS in 5G
URLLC is some how a hype as it requires link budgets in NR with at least 10 dB stronger so cell range will be much shorter.
It is anyway short in mmWave but URLLC services will have even shorter range.