In law, a reply is a legal document written by a party specifically replying to a responsive declaration and in some cases an answer.
A reply, specifically in California, may be written, filed and served, when a party files a motion or Request for an Order, the non-moving party files a responsive declaration, then the moving-party wants to file a legal document specifically 'replying' to the responsive declaration.
[1] It is important to keep in mind that "plaintiff" in this context may also refer to an impleaded party.
The court may order a reply to this third-party defendant's answer.
In California, the filing of a reply is subject to CCP 1005[1] and the reply should be filed and served pursuant to these rules - typically five court days prior to a hearing see California CCP 1005 (b).