Many are confused by the different types of events in randonneuring. That's completely understandable. The sport itself is steeped in rules and paperwork going back 100 years...
Today, there are a number of differences, subtle and not subtle. This will hopefully help people understand better what the options are and how they fit into the increasingly complicated RUSA awards program.
RUSA is Randonneurs USA. They used to give everyone a very useful booklet about the history of the sport but that hasn't been done for a long time. RUSA started about 25 years ago after the collapse of International Randonneurs (its predecessor). They are the primary certifiers of qualifiers for Paris Brest Paris in the US. Other countries have their own equivalent bodies (Audax UK, etc). Long ago, the only reason to do a brevet was to qualify for Paris Brest Paris (this is mainly the 80s and early 90s). You must be a member of Randonneurs USA to do ANY RUSA associated event. Current cost is about $30 a year.
The ACP (Audax Club Parisienne), is the most famous of the national organizations of randonneuring. Located in France, they managed and hold Paris Brest Paris every 4 years. They also hold brevets in France and allow other countries (such as the US) to hold brevets that satisfy their criteria and qualify for PBP and the awards that the ACP offers. The ACP Super Randonneur, Randonneur 5000, etc are examples of these kinds of awards. RUSA is their intermediary in the US and registers/certifies those events.
RBAs must (in general) submit any ACP events for the next year to RUSA in September/October of the previous year. Those go on the RUSA calendar but are always identified by ACP. Only by doing ACP brevets can one qualify for the French awards. These are special events and really should be considered the classic randonneuring event. They have standard finish and specific control times (13.5/20/27/40 hours) for standardized distances 200/300/400/600k. No matter the exact distance, the credited kilometers and time limits are the same. It also used to be that anything ACP could not be certified between a cut off in October and December 31 - though that has changed a bit in recent times.
Randonneurs Mondial is the worldwide body that certifies 1200k and above events. They set standards for the time calculations for those events. The ACP does use its own modifications for PBP - you can do that when you are the oldest group in the world. The list of those requirements can go on for pages and can change too!
As more people became interested in Randonneuring, RUSA expanded options. There were permanents. These are managed by RUSA and initially a permanent could only be riden once per year by a rider to qualify for distance in RUSA per year and the R-12 (200k every month for a year). RUSA also started offering its own awards for its own events (though they credit any ACP event as well). It's a rule that any RUSA award can be achieved by any member too - so there is no "Kilometer King". Permanents are managed separately and now require an additional fee to ride them. They used to be open to everyone and were managed by any individual RUSA member. That changed about 3 years ago. Permanents can be ridden by anyone subscribed to the permanents program on their own schedule.
There are also RUSA Brevets. These are NOT certfied by the ACP, but can be scheduled no less than 6 days before the event. The time limits are a little different and the overall time is based on the actual distance instead of the standard (like ACP events). This provides for different distances that are better for an odd number such as 250k or 500k - it would be very hard to do 250k in 13.5 hours so it expands options. The 6 day limit also makes it a lot easier to accommodate bad weather or early brevets that might be contingent on outside circumstances (or are outside the time limits for the ACP).
RUSA awards include (and seem to expand all the time)
Many people now refuse to ride anything if it doesn't contribute to a RUSA award. More people are mixed and have one as a goal. Some could not care less about them. All RUSA awards must be applied for on their website. If you want medals, you can buy them through the RUSA store.
The major difference between all of these types of events is who certifies them and what awards they qualify one for. In general, the
ACP
RM
RUSA