UCS-2 is what a Unicode implementation was up to Unicode 1.1, *before* surrogate code points and UTF-16 were added as concepts to Version 2.0 of the standard. This term should be now be avoided.
When interpreting what people have meant by "UCS-2" in past usage, it is best thought of as not a data format, but as an indication that an implementation does not interpret any supplementary characters. In particular, for the purposes of data exchange, UCS-2 and UTF-16 are identical formats. Both are 16-bit, and have exactly the same code unit representation.
The effective difference between UCS-2 and UTF-16 lies at a different level, when one is interpreting a sequence code units as code points or as characters. In that case, a UCS-2 implementation would not handle processing like character properties, codepoint boundaries, collation, etc. for supplementary characters
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