What does it mean to say that a term cannot be "bargained to impasse"?

A "bargained to impasse" term is one that negotiating parties use to indicate that they do not wish to pursue a particular negotiating item

There are some terms that while negotiable cannot be the basis for industrial action. If agreement cannot be reached between parties on such terms, they must be dropped or the status quo must prevail

A "bargained to impasse" term is negotiated separately fram the main issues under contifferation

A "bargained to impasse" term means that a particular term is legislatively mandated$-$for example, the duty to bargain in good faith $-$ and so cannot be changed at the negotiating table