It depends what's customary for the particular Multiple Listing Service. As Katina said, some will indicate a property is under contract with an inspection contingency, for example.
A major contingency (especially in a slow market) might be the sale of the Buyer's home. In such a situation, and if the contract has a "back up offer" addendum or "kick-out" clause, the MLS may allow agents to keep the property "active" with the notation that there is already a contract on the property, but the Seller is entertaining back-up offers.
In my MLS, short sales (pre-foreclosures) must stay in as "active" with a notation that says "Bring back-up offers" or similar language. And, for sale signs usually stay on the property until it closes.
All the best.
Maggie Hawk, REALTOR
Watson Realty Corp.