DA-Apollo Service propulsion system ...

[Jestersage]

I think most people in this forum knows the the AJ10 of Apollo was OP because it was originally for Direct Ascent, or as wikipedia stated:

Quote

the engine was originally sized to lift the CSM off of the lunar surface in the direct ascent mode assumed in original planning

However, is the AJ10 (in the original direct ascent mode) acting as a descent engine, ascent engine, or does it do both (ie, provide some ascent power before being discarded)?

Edited April 25, 2019 by Jestersage

[Reactordrone]

Looks like all the proposals had a separate ascent stage. They were massive vehicles so staging would be very beneficial.

http://www.astronautix.com/a/apollolunarlanding.html

 

[DDE]

4 hours ago, Jestersage said:

However, is the AJ10 acting as a descent engine, ascent engine, or does it do both (ie, provide some ascent power before being discarded)?

Generally the beneficial combination is, as seen on the L3 LK, a descent stage dropped pre-landing, and the ascent stage being used for final hover and touchdown to reduce the size of the final vehicle on the Moon. The descent stage becomes known as the “crash stage”.

[magnemoe]

2 hours ago, DDE said:

Generally the beneficial combination is, as seen on the L3 LK, a descent stage dropped pre-landing, and the ascent stage being used for final hover and touchdown to reduce the size of the final vehicle on the Moon. The descent stage becomes known as the “crash stage”.

Yes it reduce the mass you have to land with and the height of the lander. You also want to stage on accent to leave legs and ground equipment behind. 
You could just drop legs and tanks, this is very common in KSP.  However having an separate accent engine is easier. It also give you an abort to orbit mode if landing engine fails.