The actual frequency error for sweeping is (reference error + 1 sample), so if you have a sweep with a “bin size” of 10 Hz (around 40 Hz RBW), and a 1 ppb timebase, your marker should read within 10.44 Hz of the actual frequency.
You can calculate bin size as (span / number of points).
If you are using the frequency difference meter or the modulation analysis tool center frequency on a CW, the frequency error should be very small, as long as your signal to noise level is good. With the frequency difference meter, I typically see something like 0.01 ppb error.