Generally it isn't. At least I have never that I can remember bought or used anything but light wall ERW
Better finishes for shop fittings and the like
It's ERW ( electro resistant welded) just a neater seam not non existent remember
In practice shs fits well if you choose wall thicknesses and radius well
What sizes you looking at and what's it doing?
Can't say I've ever had much issue with shs sliding in a bigger section as if like Kent says you pick the sizes correctly. It's never going to be a machined close tolerance but it does fine for most jobs.
Same thing isn't it ? SHS and RHS is the section, ERW is the process used to make it. Cold drawn, DOM or seamless tube in any section would be stronger but it is reflected in the price.
When I used to make furniture and light fittings we always used ERW as it cold formed more easily than CHS/RHS etc., plus it seemed to have a bigger variety of wall thicknesses in the smaller sizes <=30mm.