How does the CRediT system work?

Question

I'm preparing a submission to Royal Society Biology Letters, and they also require that each author's CRediT contribution is provided. I'm not familiar with this (I usually publish in engineering venues that do not require detailed contributions), could someone clarify a few things?

There are four options for each category: lead, equal, supporting, and not ticking the checkbox.

Question 1
I'm assuming that not ticking a checkbox is a valid entry for "no contribution" for that category. If a category is not very relevant for the study, can it be skipped entirely?

Question 2
Are lead and equal mutually exclusive? Or can they be used to split contributions into three levels? Same between supporting and equal. (Author 1: Lead, Author 2: Equal, Author 3: Equal, Author 4: Supporting 5:Supporting)?

edit: in a bizarre turn of events, when I logged back in the CRediT accreditation was no longer in the system, and instead there was a field for freetext in which to write a statement of contributions, following RS's own guidelines. No idea what happened there.

Answer 1

Journals use a variety of formats for reporting author contributions. This content is like a conflict of interest statement—it's not typically evaluated as part of the peer review, but more a contractual reassurance in case of later concerns such as whether an author is added with no contribution.

I've used CRediT statements in several published papers. I don't know a citation for my claims below, but I strongly suspect:

1. Yes
2. Not mutually exclusive