What Neptune does in a chart
Neptune is the part of the chart that answers one question:Where do I dissolve the boundaries? It describes imagination and idealism — not events, and not outcomes.
What Capricorn does to it
Capricorn is a earth sign, so it moves on evidence and consistency. It is also cardinal, which means it starts things. Put those together with Neptune and you get dissolves into capricorn themes — structured, patient, ambitious, applied specifically to imagination and idealism.
Capricorn is ruled by Saturn, so this placement tends to be coloured by structure and commitment as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.