Standard Hydrostatic tests

In UG -99 Standard Hydrostatic tests (e) 1
The second line of (1)
If the common elements of a combination unit are designed for a LARGER DIFFERENTIAL PRESSURE than the higher maximum allowable working pressure to be marked on the adjacent chambers , the hydro test shall subject the common elements to at least their design differential pressure , corrected for temp.........................
I may have this wrong so im looking for opinions on what this means, and can anyone give me an example of how to use this in a hydro test?
Please give an example of a common element.
Any input would help.
MDK

A typical example of a combination unit is Shell and Tube Heat Exchanger with Tube sheet as a common element.

Regards