Backer Blocks- are pieces of plywood or OSB that fill the gap between the flanges of the joist. These blocks provide a nailing surface for face mount hangers or help brace the top flange for top mount hangers. They are installed tight to bottom flange with face mount hangers and tight to top flange with top mount hangers. Minimum length should be 12".
Filler Blocks – are blocks constructed of thick plywood, OSB or dimension lumber to fill the gap between double joists. They are required when the double joist member is supporting load from one side. Minimum length should be 24".
Note that the height of the backers/fillers may be varied, depending on the attaching hanger height when using face mount hangers. For connection info, and additional info not shown here, see Specifier's Guide TJ-4000 (link below).
Do not attach double joists together by nailing through the flanges.
Refer also to article Double TJI joists