SessionID is a String property that represents the unique identifier for the HTTP session. SessionID is the "IDSESSIONID" Cookie value used to implement HTTP session persistence. 

SessionID is used when the GetSession method in TIdHTTPDefaultSessionList is called to locate a persistent session having the desired SessionID and RemoteHost provided in an HTTP session. 

SessionID is updated when TIdCustomHTTPServer.CreateSession is used to initialize a new persistent session for requests to the server. 

RemoteHost identifies the IP address that is the remote endpoint for the HTTP session. 

RemoteHost 

TIdCustomHTTPServer.CreateSession 

TIdHTTPDefaultSessionList.GetSession 

TIdHTTPDefaultSessionList.CreateUnique