Two kinds of travel days
The first is an individual's travel. An actor based in New York is cast in a feature shooting in New Mexico. She flies out the day before her first work day, so her day out of days row starts with T, then SW. After her WF she may get another T to fly home.
The second is a company travel day, when the whole unit relocates. A feature that shoots two weeks in Albuquerque and then moves to Santa Fe might put a banner on the board reading "TRAVEL DAY: COMPANY TO SANTA FE" between day 10 and day 11. That day isn't a shoot day, but crew on weekly deals are still on the clock, and hotels, per diem, and trucks all run.
Pay under SAG-AFTRA theatrical terms
SAG-AFTRA's help center summarizes the theatrical contract this way: when a day performer travels to or from a location on a day with no work, they are paid for a full day, and overtime caused by travel time is paid at time and a half (SAG-AFTRA theatrical travel). Crew travel pay comes from the relevant union agreement or the deal memo.
Worked example: a travel day that pays twice
Joe is a day performer with scenes on days 4 and 7. First draft of his DOOD row: T on 3, SW on 4, H on 5 and 6, WF on 7, T on 8. That is six paid days for two days of work.
The AD finds that Joe's day 7 scene (INT. MOTEL ROOM - NIGHT, 1 2/8 pages) can shoot on day 5 after the motel exteriors. The row becomes T on 3, SW on 4, WF on 5, T on 6. Four paid days, and one fewer hotel night.
Could Joe fly in the morning of day 4 and work that afternoon? If the flight is short, maybe, but then travel counts toward his work hours and a delayed flight puts the whole day's scenes at risk. Many ADs prefer the separate travel day for anyone in the first scene of the morning.
Mistakes to avoid
Leaving travel off the DOOD is the classic one, because the budget then under-counts cast days. Another is booking flights before the schedule is locked, then having to pay change fees and an extra hold when the schedule shifts. The travel coordinator should work from the latest DOOD, and the production coordinator should put arrivals on the call sheet's travel section.