There are some situations, such as those involving large junction or pull boxes that serve as central takeoff points for many-plexed homeruns or homerun banks, enclosures that form central splice points for control wiring, or situations where a panel cabinet is converted to a junction/pull box, where there are so many similar wires and splices that using free-floating wire splices edges towards the verge of no longer being "neat and workmanlike" to the standards of NEC 110.12.
While these sorts of problems are commonly encountered in the industrial controls world, the solution they use over there (namely, terminal blocks or terminal strips, whether they be of the NEMA style sometimes sold in hardware stores or of the IEC/DIN style used more commonly in industry) is not applicable for field mains wiring, as terminal blocks are only available in the form of UL recognized components (to UL 1059, Category Code XCFR2). Furthermore, UL listed power distribution blocks (to UL 1953, Category Code QPQS), while legal in this application, have several drawbacks: they are large and relatively expensive relative to normal connecting means for 14-10AWG wires, while the available configurations are not conducive to single-sided tapping and splicing or to connecting large numbers of similarly-sized wires.
As it turns out, though, there is such a thing as a mountable splicing wire connector: ILSCO (NIMBUS PBTM aka PBTD-M) and NSi/Polaris (Polaris IPLM(D)) make mechanical screw-type connectors that have mounting holes for bolting to a chassis or enclosure, and, more importantly, Wago makes DIN rail carriers for their 221 and 222 series splicing connectors (the 221-500/221-510 and 222-500, respectively). This appears to be all well and good, as the underlying splicing connectors are all UL listed (to either 486A-B for the mechanical screw splices/taps, or 486C for the lever-type splicing connectors), until you stop and check the definition of "splicing wire connector" found in UL 486A-B-C and ZMVV.GuideInfo:
Splicing wire connectors establish a connection between two or more conductors by means of mechanical pressure and are not intended to be permanently mounted. They are floating, such as a twist-on connector in an outlet box.
So, does the act of permanently mounting a splicing wire connector to an enclosure using a manufacturer identified mounting facility void the listing of the connector, putting me back where I started? Or does this situation simply mean the AHJ must make their own determination of acceptability, instead of being able to say "that's listed, so you're good?" What applications are these mountable splicing connectors even intended for, if they cannot be used in applications where a UL listing is required?