The following description appeared on the IllinoisGreensFocus email list on 2003/3/22:

Illinois DOES NOT have an "OPEN" primary. Legally we have a closed primary with "same day" party registration. Voters register as members of a political party by requesting a Primary Ballot for the respective party. The Voter is then a registered member of that party until he asks for another primary ballot in another election.

A followup message said this:

At this point, and in the future, IGP membership means absolutely nothing except for internal party matters - it has no bearing on the statutory electoral system whatsoever.