Ernst,

As long as you don't miss any support points, the shock definition need not be defined on one line of input. So, assuming your model has no other supports between 120 and 150, these two sets are the same:
-1-
Spectrum:X Factor:1 Start Node:10 Stop Node:150 By:1 Anc Move:0
-2-
Spectrum:X Factor:1 Start Node:10 Stop Node:120 By:1 Anc Move:0
Spectrum:X Factor:1 Start Node:150 Stop Node:150 By:1 Anc Move:0

However!
Note that I included the anchor movement. Leaving that off will have CAESAR II calculate this value based on the entered spectra.

The various combination methods defined in the Control Parameters will affect the total response in -2- above. If you leave the default directional combination method to SRSS, the response from the shocks will be the same for -1- & -2-. I cannot say -1- & -2- will be the same for absolute summation. Missing mass and anchor motion combination methods may cause differences between -1- & -2- as well.