Returning from what? FMLA? Mil Leave?

From previous alpa experience a WRONGUL Termination (ie you did nothing wrong) would get you back at your original seniority.

But a termination for cause where there was some sort of misconduct would often get you hired back at a lower (or zero) seniority.

I guess it depends on was your termination thrown out on review, ie reinstatement? You should go back to your original seniority, whatever that's worth in the new company.

If they just hired you back, maybe a different story.