What is a Program Work Breakdown Structure (PWBS)?
The program WBS decomposes the program to the level where components take over: program-level deliverables, the components themselves, and program-office work (governance, integration, benefits management) that belongs to no component. Below each component node, that project's own WBS carries on.
Its value is the seam: it makes explicit which work is the program's (integration test labs, shared infrastructure, transition activities) so it gets planned and funded instead of falling between components.
Worked example
A defense program's PWBS shows: 1.0 program management office, 2.0 vehicle platform (component), 3.0 communications suite (component), 4.0 systems integration & trials, 5.0 training & fielding. Elements 4 and 5 belong to no single project — which is precisely why the PWBS exists: two years later, nobody is asking "whose budget covers the trials range?"