He shakes Data's hand with a nod. "Yours, too. And I was with the RAF. You might be used to seeing service dress uniforms. This is routine working dress. Or are you familiar with the RAF?"
He lifts his left arm and gestures to the leather strap on it. "I did take them with an instrument. And when you've slipped in space and time, it's good to know if you're still in your own universe or not."
Assuming Jack is in his usual coat-light blue shirt-dark trousers, which is indeed uniform.