[justgo_withit]

I always liked this explanation from an old issue of The Quill:

Partially Structured Recruitment: This version of formal recruitment features a more flexible schedule, with events held over several days at different times to accommodate the busy schedules of today’s student. While potential new members can attend events that fit their schedules and interests, each woman must visit each sorority at least once. Partially structured recruitment includes a preference and bid day event, and involves a Quota and Total.

And the MOI gives a sample schedule that's also useful in figuring out the differences:

Event I — Panhellenic Fair event. The College Panhellenic hosts an open event or simultaneous events on campus in centrally located areas. Greeters are College Panhellenic officers, recruitment counselors and a selected group from each chapter. Displays, discussions and a schedule of events offered by the chapters are available.

Event II — Open House tours available to all potential new members.
Recruitment counselors may escort or greet potential new members at the chapter site. Potential new members sign in at each event so that a record of attendance is kept.

Event III — Chapters have events during the week, with events of their choice (two to three events). Events are scheduled for potential new members’ convenience (after classes, weekends and evenings).

Event IV — Invitational Preference events are scheduled.
Chapters call or give invitations to their guests. Guests may attend a specified number of events determined by Panhellenic.

At the conclusion of preference events, all potential new members are given the opportunity to sign an MRABA form with their choices in order and submit it to the College Panhellenic. Bid matching takes place.

Event V — Bid Day.
The College Panhellenic determines the time and place for giving bids.