Board Thread:Site Problems and Appearance/@comment-10771297-20151105020311/@comment-5475153-20151109022352

We've always used "recurring characters" to refer to those who make multiple appearances within the series.

If you look on the "supporting characters" category page, it says what it's for right there: "series regulars outside the main cast." In other words, they play a significant role in the series, but they aren't part of the official main cast.