Is this the language you are referring to? "Where mechanical exhaust is required by Note b in Table 403.3.1.1, recirculation of air from such spaces shall be prohibited. Recirculation of air that is contained completely within such spaces shall not be prohibited. Where recirculation of air is prohibited, all air supplied to such spaces shall be exhausted, including any air in excess of that required by Table 403.3.1.1."
Note b. "Mechanical exhaust required and the recirculation of air from such spaces is prohibited. Recirculation of air that is contained completely within such spaces shall not be prohibited (see Section 403.2.1, Item 3)."
Ok, doing my best to convert this over to plain language using a smoking lounge as an example. A smoking lounge requires mechanical exhaust per Note b in Table 403.3.1.1. It is not permissible, per the first sentence of 403.2.1(3) to recirculate this air to other parts of the building. The second sentence does allow for the recirculation of the air if that air (presumably contaminated) is completely contained to that space (i.e. a dedicated HVAC unit).
2015 IMC Commentary is pictured below...