I think there's two important ways to think of them:
these are the UTC implementations; when you need to know the time now, precisely, these are the current time references you can use. Actual UTC will get worked out later. (Thanks PM 2Ring and Johny Doty)
these are subsets of UTC time which track UTC time but which may differ from UTC on the order of nanoseconds. Various groups have to run the various clocks that are ultimately combined to make TAI and UTC. Per BIPM:
Physical realizations of UTC – named UTC(k) – are maintained in national metrology institutes or observatories, which contribute to UTC by sending their clock data to the BIPM. Values of [UTC – UTC(k)] at five-day intervals are published in the monthly BIPM Circular T, For those institutes participating in the CIPM MRA, their results are also published in the form of a key comparison, CCTF-K001.UTC, resulting in a monthly publication of the degrees of equivalence with respect to the reference time scale UTC.
NIST defines UTC(NIST) thusly:
The National Institute of Standards and Technology (NIST) maintains its own representation of UTC, called UTC(NIST), which serves as the official time standard for the United States. This was codified into law by the America COMPETES Act of 2007 which states that the official time for the United States is Coordinated Universal Time (UTC), as “interpreted or modified by the Secretary of Commerce, in coordination with the Secretary of the Navy.” Because NIST is the division of the Department of Commerce responsible for maintaining the U. S. standards of physical measurement, the passage of this bill formally established NIST as the official U. S. timekeeper.
It goes on to state that UTC(NIST) is maintained by typically 10 to 15 atomic clocks in Boulder, CO, whereas UTC is maintained by roughly 400 clocks around the world.
UTC(USNO) is maintained by the US Naval Observatory with their own set of frequency standards.
UTC(SU) is the Russian national time scale.
UTC(BIPM) is the official UTC time as coordinated by BIPM.
I'm honestly not clear on what "Europe Lab" is--perhaps UTC(ESA) kept by the European Space Operations Centre?
UTC(CRL) is the Japanese time standard.
UTC(NTSC) is maintained by the National Time Service Center (ex Shaanxi Astronomy Observatory) of the Chinese Academy of Sciences, and is also the basis for Beijing Time.
BDS apparently refers to the time standard for the Chinese BeiDou navigation satellite constellation (BDS). COMPASS/BeiDou Coordinate and Time Reference Systems
National Academies of Sciences, Engineering, and Medicine. 2012. Global Navigation Satellite Systems: Report of a Joint Workshop of the National Academy of Engineering and the Chinese Academy of Engineering. Washington, DC: The National Academies Press. https://doi.org/10.17226/13292. mentions that it's "linked to the national UTC(k)" which I infer is NTSC, so I guess that's why RINEX 7 is (NTSC)(BDS), even though I think they'd technically be distinct.