The Category Numbering system used to identify each set and each item within a set on this website follows these rules :
Each set issue will have a unique number
Xnnnn where X =Card category and nnnn = Set Number
Eg. 
Single issue items (Sets that contain only one item) will have a set number defined as
XSnnn where X= Card Category, S denotes single item issue and nnn = Set Number
Eg. 
Each individual card will then have a sequential number within the set, so an individual card catalogue number would be Xnnnn-ccc.
Eg. 
Error cards will appear with an ‘e’ following the individual card number.
Eg. 
Reverse styles within a set are numbered Xnnnn-rev1, rev2, ……
Peripheral items issued with a set, such as an album, a wrapper, a box etc, will have a non numeric category number Xnnnn-alb, Xnnnn-wrap, Xnnnn-box, etc.
Membership Cards and Badges
The Catalogue number takes a slightly different form for these items
Xnnnn where X =Category and nnnn = Set Number
Each individual card or badge will then have a year number within the set, and a team indicator. An individual item catalogue number would be Xnnnn-yyyyttt.
For example a 1908 Fitzroy Membership card would be M2-1908fit.
Membership cards and badges can have variations within a single year, so a single letter indicator at the end of the Catalogue can be used to show this.
For Example a 1908 Fitzroy Junior Membership card could be M2-1908fita.