The header for an answer needs to include its score, as that's necessary for people to compare the answers. For most code-golf answers, this will be the length in bytes.
It's also helpful to include anything else in the header that might be useful to someone trying to assess the score themself. For example, if the byte count is higher than the character count due to multibyte characters, something like "6 bytes, 4 characters" or "6 bytes of UTF-8" would help to explain to people reading the code why the byte count was correct. (Sometimes it's helpful to give an explanatory note even when the byte count equals the character count; things like "10 bytes in Jelly's codepage" or "4 bytes of ASCII" are helpful to people's understanding in languages which use many non-ASCII characters, or languages which are normally encoded using sub-byte encodings but in this case are using a single-byte character set instead.)
For a code-challenge you'd normally include the score in the header rather than a byte count, but code challenges frequently have tie-breaks, and program length is a common tie break. So including that in the header is helpful if it seems like there's any chance that there might be a tie. (For example, if you've maxed out the primary score criterion, answers from then on are going to be competing on the secondary score criterion, so you'd want to list the length in much the same way as you would for code-golf; the challenge has effectively turned into "golf this program while maximising the primary score".)
This doesn't apply when an answer has no readily-available score (e.g. because it's an uncracked cops-and-robbers submission and the secondary scoring criterion is something that can't be revealed until the answer becomes safe, or a king-of-the-hill submission whose score may change from run to run).