In electric circuits, we have conductors (copper) and insulators (plastic). Actually, we have a broad range: copper is very good, iron less good, but any metal is many thousands of times more conductive than plastic, dry wood, etc.
In magnetic circuits there are only two materials:
1) Iron
2) anything else
Empty space, air, wood, brass, animals minerals and vegetables are all "the same". We define the magnetic conductivity of empty space as "1.000000", and these other materials run 1.0000 to 1.001 or so.
The main exception is iron. Iron has magnetic conductivity around 1,000, a LOT more than air or brass. Wave a magnet around the world, and there is no doubt when you hit some iron.
Small exception: Nickel and some other metals and alloys are magnetic, just not as high as iron. Stainless Steel can run from not much more than 1 to way over 100. (And generally, the "better" it is for stainless-ness, the less it sticks to a magnet: really-stainless steel has so much Chrome in it that it breaks-up the iron magnetic domains and seals the raw iron away from air and water.)
When reading a magnetic circuit, ignore all the non-iron to get the basic shape. THEN go back and remember that all the air/brass/etc also conducts magnetism a little. 1/1000 as much as iron, but there is air everywhere: a small leakage over a large surface is a significant shunt on the intended magnetic-gap.
