"The Free Syrian Army (FSA) has confirmed it has retreated from Salah al-Din, a densely packed area of narrow streets on the south-west side of Aleppo, where rebel fighters had been heavily dug in. State media had reported the army was in full control of the district, saying it had inflicted heavy losses on hundreds of "terrorist mercenaries". "We have staged a tactical withdrawal from Salah al-Din," rebel commander Hossam Abu Mohammed of the Dara al-Shahbaa Brigade in Aleppo told the AFP news agency by phone. "The district is completely empty of rebel fighters. Regime forces are now advancing into Salah al-Din."" BBC
The Beeb seems like a good source on this. A lack of ammunition re-supply and exhaustion seem the most likely problems for the FSA.
IMO the government will proceed to clear one district after another. In Damascus they then went house to house looking for their adversaries.
If they follow traditional patterns in a kesselschlacht like this, they will either make a tight cordon around the city to try to intercept fighters leaving the city or leave a corridor through which the enemy can run a "gauntlet of fire" trying to escape. This latter course of action is always tricky since the enemy may escape in large numbers if the corridor is too wide. the US Army learned that lesson in 1944 when the Falaise Pocket was insufficiently closed off. pl