A SMURF code is a Spider-Man Universe Reading Flow code. It is of the form of a pair of three digit numbers NNN.MMM which define the reading continuity typically relative to Amazing Spider-Man. Usually, Amazing Spider-Man receives the 500 sub-code. e.g. The SMURF code for ASM #207 is "207.500". The best-recommended reading continuity is then in numerical order of SMURF code.
In general, no two issues should have the same SMURF code, unless the two stories interweave to the point where they effectively take place at the same time. Note that the SMURF code for a backup story will be different than for the main story in an issue. Variant covers should inherit the same SMURF code as the standard cover. Issues which have no correlation to the "mainstream" Marvel Universe are not covered by this system.
We are currently in the process of setting up our SMURF code system, and we have a "work in progress" Spider-Man Continuity Guide. Please Mail Us if you are willing to help enter SMURF codes for issues which do not currently have them.