The February ceasefire in Syria suffered greatly from the Nusra Front not being a party to the truce but being embedded with almost every rebel group that was a party. This led to constant claims Syria was “violating” the ceasefire by targeting Nusra, even though they were supposed to be able to, and left Nusra with a lot of extra territory when the truce collapsed.
This new truce, while again excluding Nusra, is supposed to be different, with the US demanding all of its rebel allies to totally cut ties with Nusra immediately. Sources within the Syrian military, however, say that so far there is no sign of that happening.
Throughout the Syrian Civil War, the rebels have often relied on Nusra to do the heavy lifting in joint offenses, and have long resisted the idea of splitting with them, insisting it amounts to a Russo-Syrian plot to divide the rebellion. The big change, then, is that publicly the US is no longer supporting this narrative, and instead demanding the split.
Whether the rebels actually start abandoning Nusra or not may not be as significant as that shift, and assuming the US follows through with its promise to engage in joint attacks with Russia against Nusra, they may quickly find that distinguishing between “Nusra targets” and other rebels is, as Russia has often noted, not so easy.