From the Ten-Tec Reflector November 1, 1999
200's are for accessory items - antenna tuners, filters, dummy loads, etc.
300's we have used for "connecting" items - cables and remote knobs.
400's are linear amplifiers
500's are transceivers
600's are keyers and keying devices
700's are microphones and SSB related devices.
800's Scout modules
900's are power supplies
Over the years, a few items have strayed from this (like the use of 200 series numbers for older power supplies/mics) but it has been fairly consistent for the last 10 years or more.