Tier (i) contains the most popular systems; posts about them are common on raif, and even the least-used one has at least a game or two each year. It contains Hugo, Inform, TADS, and ALAN. These are all good systems, with Inform and TADS the most popular and ported.
Tier (ii) contains systems that are either waning in popularity, or have not started waxing yet, though they are being supported by authors. It contains AGT, Quest, SUDS, ADRIFT and PAWS.
Tier (iii) mostly consists of old systems that never really caught on.
The Unprocessed Tier lists new or previously overlooked systems which should be listed in one of the other tiers but haven't yet been added.