snc/extid_login_rfc is a SAP Parameter attribute which is used to control Enable login with external identity (RFC) information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter must be set to 1 so that it is possible to log on with an
external identification, for example, using an external server (such as
the Internet Transaction Server ITS). This is not permitted by default.
This parameter relates to RFC log-ons. -
(Further prerequisites for accepting an external identification is SNC
secured communication to the server and targeted the release of the
server for this logon variant)
See SAP Parameter Documentation for full SAP documentation for this profile parameter.