I would say the line that represents the eastern border of states that actually have a coastline to the west.  Technically there are geographic borders to what is considered the "west coast" - the Cascades, Sierra Nevada, and the Mojave Desert, but people who live in the 3 coastal states are usually considered to be from the west coast, even if they live east of those markers within those states.

Utah has long been considered one of the 8 Mountain states (aka Mountain West).  https://en.wikipedia.org/wiki/Mountain_State  Normally California, Oregon and Washington are considered the West Coast states, although for US Census purposes, Hawaii and Alaska are included with those states as the Pacific states https://en.wikipedia.org/wiki/West_Coast_of_the_United_States