Definitions

Term	Definition
Callsign	The unique identifier that a client is logged in as; e.g. `AAL123`, `EGLL_TWR`, `XX_SUP`, etc.
Client	A pilot or ATC station. For the purposes of FSD, observers are considered to be ATC stations.
Client string	A string identifying the client software. This is the FSD equivalent of user agent strings for browsers. Examples include `Euroscope 3.2`, `vPilot`, etc.
Frequency (freq)	The network analogue to VHF frequencies used for aircraft radio communication. For the purposes of FSD, observers are always tuned to 199.998 MHz.
Latitude (lat), longitude (lon)	Latitude and longitude. They have a precision of 5 decimal places.
Network ID	A unique ID used by the network. For VATSIM, it is the CID; for IVAO, the VID.
Protocol version	The version of FSD being used by the client. As of the time of writing, VATSIM uses `100`, while IVAO uses `B` (all clients except IVAC 2) or `C` (IVAC 2).
Rating	The ATC or pilot rating, as assigned by the network. This is stored as an integer; the lowest rating is `1`. Examples: S1/C2/C3, AS3/ADC, FS3/SPP, etc.
Server	A FSD server.
Packet	A formatted unit of data sent over an FSD network.

Placeholders are indicated using brackets; e.g. (client string).

Ratings

Each user has a pilot and ATC rating.

This is stored as an integer inside cert.txt on the server, but is advertised by IVAO and VATSIM under the following names: