It seems to me there are relatively few vaulable Stealth skill feats available, but Terrain Stalker looks like a good choice.
However, Terrain Stalker tells you to choose "rubble, snow or underbrush", without actually defining what these represent. I would've liked to have some NPCs select urban terrain, for sneaking through refuse-sterwn back alleys, but urban terrain is not on the list.
Would it be reasonable to qualify cluttered urban terrain (garbage, barrels, boxes, etc) as "rubble"? Or is this trying to reach beyond a reasonable interpretation of this feat?
Since it's very GM dependent, the only appropriate answer is to ask your GM. However, I ruled that if in a city and there was difficult terrain, the chances are high that I would classify it as rubble.
The feat name is slightly misleading to those of us used to the broader environments of terrain in PF1.
Rubble, snow, & underbrush depend on the square your PC is in (or arguably behind). So you'd likely find your answer on a battlemat/flip-map key or in "theater of the mind" you'd have to ask about the prevalence of the one you like.
So in an old growth forest, you might get tons of underbrush, while in a forest recovering from a fire, there may be little. Meanwhile an orchard with the same number of trees may give you zero opportunities.
I wouldn't expect any of those options to work well in a city south of the snowline. Since they're all terrain you can hide under, I think any replacement would have to be similar. Could I get underneath the terrain I want to suggest to my GM?
(I'm a bit surprised there isn't an aquatic or swamp/mud option.)
My impression is that "underbrush" is the most generic of the terrain options, since any forest, swamp, prairie or hill area is liable to have at least some underbrush, and maybe a lot of it.
I'm tempted to interpret "rubble" terrain as any disorderly but man-made cover, so a cluttered back-alley with assorted barrels, boxes, trash and offal middens could qualify as "rubble", and not just ruins and assorted blocks of stone.
But I would've prefered to have the list of terrain types expanded a bit. If "urban" terrain were included, you could claim that a crowded market or even a busy street could qualify as cover-worthy terrain, for the purposes of this feat.
In stories, the kind of Terrain Stalker hiding used in cities involves hiding in a garbage bin, under an overturned crate, clinging to the underside of a wagon, or even holding one's breath in a water barrel. I would describe that kind of difficult terrain as "clutter."
I view rubble as boulders, rocks, and broken ground such as gullies. A city could have rubble, such as a fallen wall, and I would even allow a trash heap to count as rubble.