Top-down and bottom-up design strategies in PWC

First heard about 'Top Down' design around 30 years ago from a System Builder
(that's what we called panel builders then) salesmen. Nice man, but what did it mean...
whatever your client wanted it to be.
For a System Builder, it was client specific requirements (later known as URS) layered
down into such detail (later known as FDS or functional specification) that software could
be coded. One client insisted that as his process in the Nuclear Industry was so complex,
coding could not be achieved until Flowcharts were drawn up - all part of Top Down design.

So is 'Bottom Up' design the opposite of the above ?
The nearest I could get is where the main contractor, not the client determined the
process. When it came to Commissioning, the process did not work quite as expected.
Re-writing software on site is a challenge but can be done - and it's where the
Control Engineer runs the process not the Process Engineer, at least for while !