Evidential describes something that can be used as evidence, or that has to do with evidence — a paper, a recording, a fact that could support a claim in court or in a serious argument. It is a formal, mostly legal or academic adjective; ordinary people say that's evidence rather than that's evidential. Though actually, it is not the same as evident. Evident means clear, easy to see. Evidential means of the kind that can prove something, even if it is not obvious at all. In language study, some teachers also use evidential as a noun for a small grammar marker that shows how the speaker knows something (saw it, heard it, was told it). That use is specialist; the meaning you will meet first is the adjective about evidence.