Is there any difference between "an epaulet" and "shoulder board" when speaking about military uniform?

Solution 1:

From Wikipedia:

Epaulettes are fastened to the shoulder by a shoulder strap or "passant", a small strap parallel to the shoulder seam, and the button near the collar, or by laces on the underside of the epaulette passing through holes in the shoulder of the coat. Colloquially, any shoulder straps with marks are also called epaulettes.

Also from Wikipedia:

A shoulder mark, also called an epaulette, shoulder board, rank slide, or slip-on, is a flat cloth sleeve worn on the shoulder strap of a uniform. It may bear rank or other insignia.

So:

The shoulder strap or passant is part of the primary garment.

The epaulette, rank slide, or slip-on is attached to the shoulder strap or passant.

The shoulder board may be attached to the shoulder strap or passant, or it may be attached directly to the primary garment, without a shoulder strap or passant.

Solution 2:

What I've noticed is that shoulder boards are stiff flat boards, shoulder marks/slip-ons are the slides that, well, slide on to the strap. Epaulettes would be the fringed accessory. I've heard epaulettes being used for all three.