As ever here in the UK, the convention for listing dimensions of solid wood, or veneered boards, the first dimension indicates the length parallel with the long grain, then cross-grain width (breadth) and lastly thickness, so generally, but not always, the long grain of the board (solid or veneered), runs left to right. It gets a bit trickier with stuff that doesn't have grain pattern grain, e.g., MDF, OSB, etc, But here again the convention I'm familiar with is that the first measurement listed is the one that runs left to right in the cabinet opening, followed by width X thickness. Of course, not all shelves are longer left to right as seen from the front and the width, which relates to the internal depth of the cabinet, is greater than the length.
It is relatively easy, even in professional workshops, for confusion and mistakes to mistakes to happen because of that, so good communication between those working on a project really is necessary.
Then, of course, there is the convention in North America, where they typically list dimensions in the reverse order to what Brits are used to, i.e., solid wood is listed thickness X width (breadth) X length (long grain dimension). It can all get a bit confusing and even chaotic, so again, good communication usually helps, e.g., if, as a maker, you're not sure, ask the designer, and if you're the designer it's quite easy to draw grain direction marks on the plan and/or include in the written notes guidance on the terms or conventions are in use. Slainte.
