From the Summary: "Nominations that have been neither confirmed nor rejected by the Senate at the time the Senate adjourns sine die or for a period of more than 30 days are returned to the President pursuant to Senate Rule XXXI, clause 6. 'Pro forma' sessions held during a recess of the Senate count as days in session and can prevent what would otherwise be a greater than 30-day recess that would trigger the return of nominations under the rule. The use of 'pro forma' sessions in modern Senate practice means that the need to suspend Rule XXXI usually only occurs at the end of the 1st session of a Congress. The Senate routinely holds over at least some nominations between sessions of Congress or recesses lasting more than 30 days. Nominations can be held over if the Senate agrees, by unanimous consent, to suspend the rule. [...] Nominations chosen to be held over are typically negotiated by party and committee leaders[.] [...] Nominations for which the rule has been suspended remain in status quo, meaning they continue to be pending on the 'Executive Calendar' instead of being returned to the President as required under Senate rules. The Senate returns all nominations at the end of a Congress. If a nomination is returned to the President, it is no longer eligible for consideration by the Senate. The President may submit a new nomination, either for the previously returned nominee or for a new candidate."
CRS Report for Congress, R46664
Congressional Research Service: https://crsreports.congress.gov/