Crawl frontier
The crawl frontier is the queue of URLs a crawler knows about but has not yet fetched. URLs enter it through links on already-crawled pages, sitemap entries, direct submissions and redirect targets. A URL that never enters the frontier is never crawled.
The frontier is not a first-in-first-out queue. It is prioritised, because a crawler has far more known URLs than it can fetch and has to choose which are worth fetching next. Popularity, how often a page has changed in the past, and how recently it was last fetched all feed that choice.
This is where orphaned pages die. A page with no inbound internal links, absent from the XML sitemap and never submitted through a tool simply never enters the frontier. Nothing is penalising it — the URL is unknown, which is a different and more fixable problem than being judged poorly.
The practical test is one you can run: if you cannot reach a page by following links from your homepage, and it is not in your sitemap, assume a search engine cannot reach it either.