The "basic rates" is the lower limit while the "supported rates" is the upper limit. Your wifi interface will not connect at a rate lower than that set on the "basic rates". Neither will it connect above the supported rates.
These are really to do with the (rf) modulations, the rates in MT is really = to the various modes,
e.g. mode 1-8 in OFDM (a/g), which translates to an average data rate in mbps (air side factors will apply).
basic-rate is used for broadcast and multicast packets (which are not ACKed and therefore must be sent at an reliable rate).
basic-rate is also used for association and beacons.
supported-rates are simply used for unicast packets (which are ACKed)
for each basic-rate there must be a supported-rate but not (necessarily) the other way around.
at "default" settings the basic-rate will be at the lowest rate for that mode (B and B/G = 1mbit, A or G-only = 6mbit) but supported-rates will vary on the success of transmission on each rate.
Basic rates are the one that device must be able to use on current tx/rx signal else it won't be able associate, supported rates on the other hand are speed rates that can be used to communicate as i was told , keep basic rates on b mode 1mb and a/g to 6mb and supportes as you think is needed.