@timtax  I'll add to Bill's response.  It does get more complicated than that.  Even though the exception rules were simplified when the proposed regulations were updated in 2013, analysis is still required because the extent to which the gain may be subject to NIIT would be determined as if the partnership had sold all its assets at FMV immediately prior to the disposition of the partner's interest.

Whether the gain/loss from the deemed sale is considered NII would, in turn, be dependent on, inter alia, whether the partner had materially participated in each of the partnership's ToB, whether the ToB is in the trading of financial instruments or commodities, and whether there are sale of assets (e.g. marketable securities) that may generate NII.  Recognizing this could be onerous for partners (and S-corp shareholders) with minute interests in those PTEs, the not-so-new regulations also provides a simplified method for determining the extent to which gain/loss may be subject to NIIT provided certain thresholds are met.

You may like to take a look at Treas. Reg. 1.1411-7 for the details and examples.