There are three types of backcountry camps within Mount Rainier National Park [1].
Trailside camps are located every 3–7 miles along backcountry trails, including the mountain-circling Wonderland Trail. Trailside camps all contain a nearby water source and pit toilet, and five people (in two tents) are allowed at an individual campsite. The vast majority of hikers use these established campsites.
If you choose to camp away from these, you’ll need to stay at cross-country camps, hiker-chosen sites located a quarter mile away from the trail and other camps and at least 100 feet from water sources. Hikers are not allowed to establish cross-country camps along the Wonderland.
Climbers generally curl up in alpine camps, in areas above 6,000 feet. The rule here is that tents must only be set on permanent snow or ice or on bare-ground areas previously used as a campsite. Don’t clear new tent sites or build windbreaks on rocky areas. The rocks you pull up could be protecting fragile alpine plant root systems.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/mount-rainier-national-park