I'm a bit confused by the different terms:
Is that correct?
Yes, that's absolutely correct.
"Symbol" is a generic term used to designate the name of an instrument. A “RIC” (Reuters Instrument Code) is a type of symbol used by Thomson Reuters. ISIN, CUSIP and SEDOLs are other types of symbols. Thomson Reuters services use RICs to identify instruments. Services published by other provider applications may use different symbol types.
A "symbol list" is an unordered list of strings. To each symbol you can optionally attach a field list (rarely used feature).
Most often the strings in the list designate data item names on the same service as from which you obtained the symbol list itself. However, there are no formal requirements of what a symbol list contains. It is just a list of "something". Whoever created the feed will let you know what the strings in the list represents. I think this feature was given the name "symbol list" because it is sort of a generic term. 99% of the time you can think of a symbol list as a "data item name list".
Data item names on TREP and in TREP APIs are really convention agnostic. They do not at all have to follow the RIC naming standard. If you have a Thomson Reuters feed on your TREP then that one will use the RIC naming standard, but other feeds on your TREP may use a different standard. It is all up to the publisher. This is why the words "RIC" and "data item name" are not interchangeable.