Data Sim were created when Blackberry and 3G services started taking off in SA. The main reason for this service were for people who wanted to have a normal voice phone together with a Blackberry and a 3G modem, but did not want to have to switch their sim between devices every time. The Twin Sim service were not sufficient for this as only on sim could be active at a time on the Network. When you have a normal contract with VSP, (Top Up Contracts and prepaid cant have the data sim service last time I checked), and you request lets say a Data Sim for your 3G modem, then you are issued with a second sim that is Linked to the account, but issued a different cell number (this is what allows both sims to be active on the network at the same time). The second sim is then provisioned for data only (no voice calls etc). You are then free to activate data bundles as per normal, wether it be a montly repeating bundle, or ad-hoc once off bundle. The Data Sim will then run off that data bundle linked to your account.
So now you can talk on your phone and browse the web all at the same time...no more switching sims to use the data bundle residing on your main account.
PS: All other standard rules will apply to the data sim...so if you have a standard data bundle, then OOB rates will apply one bundle is depleted. If you have Advanced data bundle active, then the same rate will apply as per the Advanced Bundle rules.