Short answer
"Worldwide" and "US Only" are two separate job feeds, not one feed with a filter. US clients' jobs appear in both — so if you target US clients through the Worldwide feed, you're competing against freelancers from everywhere and missing the lower-competition US-Only pool.
How to choose
Want US clients? Set the campaign's region to US Only. It's a smaller, less crowded feed of US-based clients — usually higher value and easier to win.
Want global reach? Use Worldwide — but remember it includes US jobs too, alongside everyone else's.
Want both? Run two campaigns — one US Only and one Worldwide — so each is tuned to its feed.
What actually lands a job in US Only
A job goes to US Only only when Upwork's own listing blocks off-country applicants — the client restricted hiring to US-based freelancers, and a non-US account can't submit a proposal to it at all.
If a client just prefers US freelancers but still accepts applications from anywhere, Upwork shows that as a "Preferred qualifications" note, not a hard rule — anyone can still apply, so the job stays in Worldwide.
So a job description asking for "US-based only" doesn't by itself mean the posting is restricted — what matters is whether Upwork actually stops a non-US freelancer from applying. Lancer files each job by that distinction, not by the wording in the description.
UK Only (for UK-based accounts)
There is also a smaller UK Only feed: jobs where Upwork blocks applicants from outside the UK. Only a UK-based Upwork account can bid on them.
The UK Only tab in the campaign's Region picker (and in the bulk region editor) is shown to every organization, like US Only. You can only pick it (and bid it) with a UK account connected; Lancer checks for a matching account when you link one.
A UK Only campaign needs a UK account linked to it; the automation screen tells you if you don't have one. A US account can't bid UK Only jobs, and a UK account can't bid US Only jobs.
Volume is low by nature, so the forecast for a UK Only campaign is an estimate and the usual ~100 jobs/month floor doesn't apply. Low volume is normal and not a reason to widen filters. Competition is lower, so run it alongside a Worldwide campaign rather than instead of it.
Common gotcha
Naming a campaign "US Only" doesn't make it US-only. Check the actual Region filter in the campaign's Jobs tab — if it's set to Worldwide, you're targeting every country no matter what the campaign is called.