Answer:

The terms l1 and l1' in the image of the output report above refer to the lengths of the shell with different thickness adjacent to tubesheet.

Refer to Section 13.5.9 and formulae 13.5.9-1 and 13.5.9-3.

'γ' is the axial differential thermal expansion between tube and shell.

If the shell is of uniform thickness then the values of l1 and l1' are equal to 0 as shown in the report above.