This question came up in the Online 10-day Retreat Nov 3, 2020, guided by Delson Armstrong. Day 2 was part of a daily 30-minute discussion on the suttas, the Dhamma talk and reflections.
Slightly edited to improve readability
I see that in one way, where it can be related to external phenomenon outside of the body. It can also be related to the input of the five physical senses and the sensory experiences that arise from it. So that can be the external aspect of it.
The internal is really more related to the mind, and the mental contents of the mind. But also the physical sensations that happen within the body itself  - which is in relation to feeling - for example the heartbeat,  you're feeling the blood rushing through the veins, the digestive processes, different parts of the functions that happen within the  body; that's another internal aspect of  it.
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