From Xojo Documentation
A "Pascal" string, typically used only with Mac OS 9. The first byte is the length of the string; as a result, it is limited to 255 characters. You can pass a regular String to it and it will be converted to a PString automatically. Also, PString does an implicit type conversion to String when you assign a PString to a String variable.
There may be legacy macOS APIs that make use of this data type, but otherwise it should be avoided.