To answer your question, I'd say no, because there is no
other tank feeding it. Certainly the opposite viewpoint
could be argued.

Thank you Thom, because you just gave me the solution
to a problem. An outboard tank could be modified to be
used as a day tank for a diesel engine and give you a fuel
gauge too!

I can't think of any sailboats I've sailed that had a fuel gauge.
Most of the time I've banged on tanks and judged by sound
or used a measuring stick.

Even without a gauge, a red plastic outboard fuel tank is
translucent enough to judge ift you have enough fuel
to prevent the hassle of bleeding a diesel fuel system.

I've been thinking about mounting a day tank in ECHO and
you just gave me the cheap solution I was looking for! I
just need to figure out how and where to mount one.

Thom Stewart wrote

Question Bart?

Would you consider the remote fuel tank on an Outboard Aux a day tank?

Ole Thom