UPDATE: I found this page from Symbian docs online that describes folding and collation in detail.
In Symbian jargon, the term “folded” means “case-insensitive“. If you grope throughout the Symbian docs and API looking for the term case-insensitive, you will be hard-pressed to find anything.
If you have dabbled with descriptors enough, you may be familiar with methods like Match(), Find(), Compare(), etc. By default, their operations are case-sensitive. However, there are folded variants of these methods available too, for, you guessed it, case-insensitive mode. They have an ‘F’ attached to the end of their names, such as, MatchF(), FindF(), CompareF(), etc. They work by folding the contents of the descriptors which are their arguments first, and then applying their respective functional semantics on them — or, simply, for MatchF(), it applies a case-insensitive match operation.
It is not every day that you may need to perform any operation against data without worrying about the case-sensitivity of that data, but when you do, and if you find yourself working with Symbian, you may likely quickly find yourself floundering in the API, without much or any help.
That is just Symbian.