Yep - bottom chord of an engineered attic truss from a truss company.
After reading your post, got the truss company to send over the
engineering diagrams - turns out it can support 60 psf (40 ll +
remaining dead load I'm assuming). Wasn't required for our permit,
but I'm sure the building inspector will be happy to see it in
writing.

Thanks.

If the 2x8 is the bottom chord of an engineered "attic truss", it's
probably sized for the intended loads. The other chords of the truss help
carry the load, the same way a 2x4 is typically used to build a standard
truss. The 2x8 would essentially be sized to carry the load between the
truss chords (maybe 10 feet for a typical attic truss?). Assuming the truss
is being built by a truss company, they should be able to supply you (or
your contractor) with a copy of the engineering diagrams and the loads it
is designed to carry. In fact, that's one of the required items for
building permits around here, so it may already be part of the plan
package.