Answer: Some or all the capital gain on sale of a partnership business with active participation can be subject to NIIT. If the business had no passive activity and the partner was active in the business, and the sale included only assets which were used in the business, then all the gain is entered on Form 8960-line 5c, which would make none of the gain subject to NIIT. If they sold investments or passive activities, then those gains are subject to NIIT. For a sale of interest in a partnership that is not a passive activity for the owner: “The term ‘net investment income’ means the excess (if any) of—” … “net gain attributable to the disposition of property other than property held in a trade or business” – from IRC Section 1411(c )(1)(A)(iii).
Learn more
https://www.irs.gov/newsroom/questions-and-answers-on-the-net-investment-income-tax