OK what is it?
Since the OP didn't answer:
abseil is an alternate term for rappel.
Single rope abseil means to rappel down a single line (as opposed to a doubled line) and implies doing it without a safety belay.
SRT = Single Rope Technique is the art of rappeling down (and usually climbing back up) a single line.
SRT is riskier because there's no backup - i.e. no belayer to stop a fall. If the single rope parts or its anchor fails, death is often the result.
SRT is commonly used by American cavers and has a pretty good saftey record amongst them.
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