My I understanding of the instructions are that adding the "C" is optional, but adding the "F" is a must:

Celsius entries cue I or 2 character between 0-99, optionally suffixed by
"C". Valid Fahrenheit entries are 1. 2. or 3 character integers between 0-99
suffixed by “F".

because the word "optionally" only occurs in the sentence about Celsius, and not in the sentence about Fahrenheit.