Selecting faces on bodies for named selections

[ianwhenham-bit]

I'm trying to write a script to select faces for a named selection, but am struggling with the syntax for selecting faces.
It seems to be of the format:

for face in body.Faces:

but 'body' is not defined. How do I 'define' solid bodies in the geometry so they can be searched for faces that match my criteria?

I looked at the NACA airfoil example to try to reverse understand the naming syntax, which has:
for face in fluid_faces:

Where fluid_faces is defined with a previous line:
fluid_faces = fluid.faces

Then as far as I can tell the object 'fluid' is created from the extrude command:
fluid = design.extrude_sketch("Fluid", fluid_sketch, BOX_WIDTH)

If that's the case is 'fluid' just the name of the solid in the feature tree?

Suppose I have a solid in the feature tree called ACC, how do I use the 'for' command to examine face on that solid body?

I think it's something like
for face in ACC.Faces:

But that doesn't work, with the message ACC is not defined. It seems I somehow need to assign the body to a variable?

Thanks,
Ian