Doing some research on Warrant Officers and I keep seeing that they're referred to as "highly skilled, single-track specialty officers, and while the ranks are authorized by Congress, each branch of the uniformed services selects, manages, and uses warrant officers in slightly different ways."

I understand every word of that, except for how the term 'single-track' relates to Warrant Officers or even the US military in general.

Can anyone shed some light on this?

Thanks in advance!