There's a box to check for "rest day," just above the larger box where you enter details (next to boxes for sick or injured). Perhaps you inadvertently selected it?
I see these options under "Note" but not under "Add training" which is what I used. And my entry is not headed by "Note"? Confusing to me but now I see where "rest" etc is found.
Never did pursue what "#" indicates/links/accomplishes. AP could benefit from a users' manual of sorts.