The ALPN (Application-Layer Protocol Negotiation) identifier(s).
For client sessions, this is a single string specifying the protocol the client wants to use (e.g. 'h3').
For server sessions, this is an array of protocol names in preference order that the server supports (e.g. ['h3', 'h3-29']). During the TLS handshake, the server selects the first protocol from its list that the client also supports.
The negotiated ALPN determines which Application implementation is used for the session. 'h3' and 'h3-*' variants select the HTTP/3 application; all other values select the default application.