Thank you! I just finished installing this unit and noted this empty socket. Not seeing anything to plug it into, I just went ahead and finished the install.
A little feedback on the installation instructions:
If the pipes on the indoor unit are naively bent outwards from the unit, they will not fit directly into the hole set out by the template. We had to bring it back down, and use the template (after cutting out a hole) to correctly place the bend.
The instructions do not note anywhere that the clearance required by the drain pipe on the underside of the outdoor unit needs a couple of inches extra above any solid base. We fudged an alternative with section of vinyl gutter extending out both sides of the base.
The male MC4 plug was defective with the installed pin extending beyond the housing - making it difficult to insert. Will be checking the connection with an IR camera as soon as I can