Credit cards, debit cards, and gift cards have supplemented cash in facilitating purchase transactions.  Such forms of payment are used extensively in online purchase transactions, where direct exchange of cash is usually not practical. 
The leading number of each reference number within the drawings indicates the figure in which that reference number is introduced and/or detailed.  As such, a detailed discussion of reference number 101 would be found and/or introduced in FIG.  1.  Reference number 201 is introduced in FIG.  2, etc.