"sdparta" is not a standard or common term in Firefox WebRTC session descriptions. You might be referring to sdp (Session Description Protocol), which is the protocol used to describe audio and video communication sessions in WebRTC.
In WebRTC (Web Real-Time Communication), sdp is used to convey essential information about media communication, such as media types (audio or video), codec information, and transport protocols. When establishing a WebRTC connection between browsers, this information is used for negotiating media capabilities and establishing media streams.
For instance, when establishing a WebRTC connection, one browser (typically referred to as the offerer) creates an offer that includes sdp information describing its media sending capabilities. This offer is then sent to another browser (referred to as the answerer), which generates an answer containing its own sdp information to respond to the offerer and indicate its media receiving capabilities. Through this exchange and negotiation of information, the two browsers can establish a suitable communication session.