Tim Mitchell explains what the JOIN() function does in SQL Server Reporting Services:
The JOIN() function in SQL Server Reporting Services is a handy tool that allows you to turn a list into a delimited string value. This function accepts two parameters, a list and a delimiter with which to separate the output, and returns a string with that list separated by the specified delimiter character.
It’s not an inaccurate name, but I’m surprised it’s not named CONCAT_WS() or something like that given how the term “join” has such a strong connotation in the relational database world.