Don't confuse "what gets taxed" vs "what gets withheld" while deployed.
And don't confuse "tax free zone" (no such thing) with the "combat zone tax exclusion" which is a rigidly defined, finite thing that officers with special pays (like us) bump up against.
Certain things like re-enlistment bonuses are in a special "not taxed" category if received while deployed. That is the exception everyone thinks is the rule.
Officers get a combat zone tax exclusion that can not exceed the base pay of the senior-most enlisted person in the service. Footnote 4 of the
2015 pay table states
What this means, is that if you were deployed to a combat zone in 2015, when you file your taxes in April 2016, you will get a deduction of the lesser of
1) [number of months you served at least 1 day in a combat zone] x $8119.50, or
2) your actual earned income during those months (base + special pays; not always-untaxed things like BAH/BAS/IDP/etc)
Base pay for an O3 >4yrs is 5,219.40 ... so your average fresh residency grad who gets deployed will have about $3K per month in excess CZTE room to apply toward medical special pays.
The best case scenario is that you'll receive one special pay or another while deployed, so you can at least take full advantage of the maximum CZTE. But don't get all giddy about collecting that $120K neurosurgery 4-year MSP/ISP lump while deployed, because most of it will still be taxed (albeit at a lower marginal rate than usual).