What exactly does a Mobile Vikings for Business subscription entail?

Your employee offers you a mobile subscription via Mobile Vikings for Business. You become - or stay ;-) - Viking, with your own account. That means that you can also join a Viking Clan and use Viking Deals.
Your employer will proposes a specific rate plan, based on your specific needs. Unless you get an Unlimited subscription, you are free to choose to activate a higher rate plan and pay for the difference in price yourself.
For each subscription that you activate, the costs outside the bundle of your subscription (eg for De Lijn tickets or SMS parking) will, by default, be charged to you personally. Your employer can however choose to set an allowance for you. More info about that here.