"Applicable large employers faced with the prospect of complying with the [ACA's] employer shared responsibility rules must grapple with and understand what is means to make an offer of minimum essential coverage under an eligible employer-sponsored [group health] plan to their full-time employees... Identifying an employer's common law employees in a two-party arrangement is a simple matter. But this is not always the case in three-party arrangements (i.e., those in which workers are hired from or through commercial staffing firms or professional employer organizations)....The final regulations provide a special rule governing outsourced employees or 'offers of coverage on behalf of other entities.' This rule is welcome to be sure, but it also appears to be widely misunderstood, systematically over-utilized, and in a few cases subject to interpretations that (seem to us, anyway) stray pretty far from the text."