The Kēlen alphabet as given on https://www.terjemar.net/kelen/writing.php shows variants for some letters. Here is an example:
Are there rules on when to pick which form?
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As far as I can tell, no such rules are mentioned anywhere.
However, the general orthography page only seems to mention one form for each letter, and those forms are also the only ones I can find in any Kelen word of the day entry I've looked at - granted, I haven't looked at all that many of them, so I may have missed some that use the alternate forms. It does, however, seem to me like those forms are the only ones in use.