The EXCSAT (Exchange Server Attribute) packet is sent by the client as part of the authentication process, typically as the very packet. It contains information about the client.
SRVNAM (string): the name of the client machine initiating the connection. Can be an IP address or a host name.
EXTNAM (object): an object describing various aspects of the connection, with the properties:
clientName (string): the name of the client application
clientProductId (string): an ID for the product, e.g. JCC04260300
userId (string): the OS name of the user connecting to the server
SRVRLSLV (string): the release level of the client, e.g. SQL11054 for a JDBC client
SRVCLSNM (string): the server class of the client, e.g. QDB2/NT64 for a Windows client
This packet type often contains other properties, but they are usually not relevant, except for advanced cases.
You could change the OS name of the user with
context.packet.EXTNAM.userId = 'Anonymous';