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RowID is the unique ID each item in a Repeating Section has, which can be used to reference it in macros and APIs even if the section's rows are reordered.

Preliminary testing have shown that:

RowID must be strings, exactly 20 character long, and with a mandatory leading - at the front. This leaves 19 variable characters.
Multiple - characters can appear next to each other. The following RowID is valid: --------------------
While Roll20 itself might generate RowID with mixed upper- and lower-case, setAttrs(), Sheet worker Events and attribute macros (at least) are case insensitive. If you generate RowID, it is probably best to use all-lowercase.
- TODO: confirm whether Roll20 still generate RowID with upper-case letters
While Roll20 seems to only generate RowID using the characters in [0-9a-z-], it seems any UTF-16 Code Unit is valid. The length above is measured in UTF-16 code units, as javascript does when using String.length in sheetworkers. See Mozilla Developer Network documentation on strings in javascript for details.
- For compatibility with Macros, it is probably best to not use any ASCII character other that digits, letters and dashes. While it seems possible to create a row with RowID -0123456789abcde@{} with setAttrs(), the RowID is likely incompatible with macros.
- To get a valid UTF-16 string, avoiding surrogate pairs of UTF-16 (see documentation above) might be advisable. Otherwise, care must be taken to pair them correctly.