It's not uncommon for customers to want White ink to be printed in parts of their labels - most particularly if they're using Chrome or Clear material. If you do wish to make use of White ink in this way, there are a couple of ways to achieve it during the design process, depending on which software the designer is using:
As explained elsewhere, our preferred file format is Adobe Illustrator - which is a vector-based program that produces the best printed results. If your designer is using Illustrator, the process of adding White ink is relatively simple. The designer just needs to lay out the Whiite ink on a separate layer with a custom spot color - and name the layer "white ink". It also helps if they make the "white" swatch 100% Cyan and 100% Yellow (100/0/100/0) so that the white ink areas appear bright green in the artwork and very obvious to our pre-press technicians. We will then use that layer to tell the press where to print white ink.
FYI, it's also possible to add a white layer in Photoshop - but slightly more complicated. That said, a competent designer should know how to do this.
Alternatively, if your designer is using software that doesn't support layers and/or is producing "image files" (e.g. jpg, png, or any other raster files with minimal ability for us to manipulate them) they should lay out the chosen "white areas" in a separate file and assign Black as the color (so it's visible on the screen). We will assume that any areas showing as black in that file should be superimposed in the original design and set to print White.
Also assume that anywhere there is no color defined in the artwork, no ink will be printed and the background will show through - in the case of Clear material this will be the product itself, but on Chrome it will be the shiny silver metallic material.