Construction terms can be confusing. Just like any other hobby or industry, practitioners use specialized terms to make communication precise. Roof battens are an example of this kind of specialized terminology. They're a standard part in some kinds of room and are necessary when installing roofing tiles on some buildings.
"Batten" refers generally to a thin strip of wood, plastic or metal, and is used in applications beyond roofing. In construction, these strips connect the main structural parts of a frame, often to provide bracing or similar support to both struts of an angle. A batten also is used as a shim to level spaces with a small difference between their sizes.
Roof battens tend to be long, and made of materials to match the building in which they're installed. Construction workers use them to provide a place on which to affix roofing tiles or sheets. They're not necessary found on most homes, which have sheets of plywood between the roof trusses for this purpose. Instead they're used on open roof designs where there's no real structure on top of the roof trusses.
The most common application for roof battens is to lay them at right angles to the line of the roof trusses. The roofing tiles or sheets are laid across the battens, and attached with nails, tacks or screws. How tightly the battens are spaced will depend on the size and nature of the roofing material .
A batten seam isn't a structural part of a roof, but rather a guard against leaking. On a batten seam roof, the roofer lays a batten along the seam between two panels of roofing material. The batten provides a watertight seal by pushing water to flow away from the seam. In many cases, the batten will be covered by a metal cap for further protection from the elements.
A counter-batten system lays roofing battens in two directions, both parallel to and perpendicular to the line of the roof trusses. This allows for greater flexibility when installing roofing material, and can bear heavier weights than a regular batten system. This pattern also is called a grid batten.