It says in the datasheet somewhere, cannot remember exactly where, but it is in there, somewhere. Or more likely, in their app note AN619. It is not measured in degrees. It is measured in quarter-cycles of the internal VCO.
I've been reading the data sheet for the Si5351a synth chip and still haven't found a clear indication as to the supported phase offset step size, so I'll ask here what is the step size in degrees? the data sheet can be viewed here.