Each partition was essentially a global label that was always present in the calculator's memory. A-D started execution at the top of that partition. So far, Partitions sound a lot like the labels of the earlier calculators. What made them different was that the labels 0-9 were local to a partition. This allowed the user to safely load multiple independent programs.
Partitions also lead to other conveniences.
For example, the user could print or clear individual partitions. The JUMP instruction took a partition letter followed by a local label number. B. It was even possible to call a local label in another partition by executing a GSB to a JUMP instruction. The RTN in the subroutine would return to the instruction after the GSB since the JUMP didn't affect the return stack. Indirect Addressing Indirect addressing gave the calculator, true computer power.