I think pricing unit refers to fare components.

For example, if you book JFK-LHR-TXL you may be presented with options that involve one pricing unit/fare component or two.

For one:
JFK-LHR-TXL uses a JFK-TXL fare, with a stop in LHR.

For two:
JFK-LHR is sold as one fare component
LHR-TXL is sold as a second fare component

You can most easily see this on ITA (it will return the cheapest fare, regardless of number of fare components). In the fare details you can see whether the entire journey is covered under one fare component or two.

i.e:
Fare 1: Carrier BA SHV3PXOW NYC to LON (rules)
Fare 2: Carrier BA SH5381DHR LON to BER (rules)