TRCs are used for certifying cabling systems to ANSI/TIA, ISO/IEC and IEEE standards. They are terminated with reference grade connectors per ANSI/TIA-526-14-C and IEC 61280-4-2. These standards specify multimode connectors having a mated loss of ≤ 0.10 dB. The TIA version for singlemode does not specify it at this time but can be found in ISO/IEC 14763-3, where again a reference grade connector must be used having a mated loss of ≤ 0.20 dB.
Why use a TRC? If you are installing low loss connectors, those connectors may have a loss of ≤ 0.15 dB. So testing it with a connector that has a loss greater than 0.15 dB will yield a pessimistic result. More importantly, if your link is an engineered link operating to custom test limits – you could be failing a perfectly good link. Another advantage is consistent and predictable results – something installers often complain about when certifying a cabling system.