From Xojo Documentation
The method by which the method SelectionLength will count the selection.
Choosing Characters will result in a count of the number of logical characters in the selection. For example, a emoji would be counted as a single character. Choosing CodePoints will result in a count of the number of Unicode CodePoints (basically bytes) that the characters in the selection consume. An emoji, for example, requires two bytes.
The default setting is Characters on all platforms. However, for backwards compatibility, TextArea controls on layouts created in projects prior to 2020r1 will be set to Native which means CodePoints on macOS and Windows and Characters on Linux.