It is simply:
heartwood, sapwood or less stringent barely mostly sapwood on at least one face(red, white, yellow; respectively)
Not all "Birch" lumber in N.A. is of the Yellow birch tree(B. alleghaniensis) Example already given: Sweet birch (B. lenta) is often sold as undistinguished from Yellow birch.
"Sweet birch wood is quite similar to yellow birch (*). Lumber and veneer of the two species often are not separated in the market, although production of yellow birch far exceeds that of sweet birch. Sweet birch is used for furniture, cabinets, boxes, woodenware, handles, and millwork, such as interior finish and flush doors." (*) Brisbin, Robert L., and David L. Sonderman. 1973. Birch ... an American wood. USDA Forest Service, FS-221. Washington, DC. 11 p
And from 2007 The Encyclopedia of Wood, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory "Characteristics and Availability of Commercially Important Wood, Cha. 1 pp 4-5 (U.S.):
"The three most important species are yellow birch (Betula alleghaniensis), sweet birch (B. lenta), and paper birch (B. papyrifera). These three species are the source of most birch lumber and veneer. Other birch species of some commercial importance are river birch (B. nigra), gray birch (B. populifolia), and western paper birch (B. papyrifera var. commutata). Yellow, sweet, and paper birch grow principally in the Northeast and the lake States; yellow and sweet birch also grow along the Appalachian Mountains to northern Georgia.
Yellow birch has white sapwood and light reddish-brown heartwood. Sweet birch has light-colored sapwood and dark brown heartwoo tinged with red. For both yellow and sweet birch, the wood is heavy, hard, and strong, and it has good shock-resisting ability. The wood is fine and uniform in texture. Paper birch is lower in weight, softer, and lower in strength than yellow and sweet birch. Birch shrinks considerably during drying.
Yellow and sweet birch lumber is used primarily for the manufacture of furniture, boxes, baskets, crates, wooden ware, cooperage, interior woodwork, and doors; veneer plywood is used for flush doors, furniture, paneling, cabinets, aircraft, and other specialty uses. Paper birch is used for toothpicks, tongue depressors, ice cream sticks, and turned products, including spools, bobbins, small handles, and toys."