A party is in default when the party fails to make a response to a petition/lawsuit in Missouri. Other than the obvious downside of a default judgment, a defaulting party is not entitled to all notices when in default, either. Under Rule 43.01(a), parties are to be served with every pleading subject to the original petition — but no service need be made on parties in default for failure to appear, except that pleadings asserting new or additional claims for relief should be served.
When there is a new or additional pleadings against a party in default, the party in default is to appear and defend within the same time as is required after the original service of process like character. Rule 55.25(d).