Timing specifications: can a spec be converted from units of +/- ms to parts per million?
Your specification gives an absolute accuracy for a range of pulses.

This means in turn, if you convert it to a relative accuracy like ppm (or %) you get a range of accuracy. This is quite unusual and would probably lead to difficulties interpreting the specification.

With your example you'd end up with:

\$0.1~\text{ms} / 1~\text{ms} = 0.1\$ or 100000 ppm

\$0.1~\text{ms} / 50~\text{ms} = 0.002\$ or 2000 ppm

So with these values, how would you accurately describe the specification you have now?

Accuracy: 100000 ppm going down to 2000 ppm in a reciprocal way according to the set period. (don't know if that's even a correct expression there)

Or maybe "2000 ppm at 50 ms" but that leaves room for interpretation as well.

It gets a mess. So you might be tempted to just choose the stringent measure, which would be 2000 ppm.

Someone implementing this would end up creating a system which is capable of creating pulses with +/- 2 µs because the worst case scenario is 1 ms and 2000 ppm.

Now that is a lot more difficult than a system with +/- 0.1 ms.

So for this scenario a relative accuracy doesn't hold well. If you have to do a worst case analysis, with this specification you'd get 100000 ppm.