When ordering lever binders or ratchet binders, notice that there are 2 measurements for the size noted.  There are two main grades of chain for tie-down applications, Grade 43 High Test Chain & Grade 70 Transport Chain.  The 2 measurements marked on these binders are telling you which size chain to use with which grade.  In an example of 5/16" x 3/8" Lever Binders, the 5/16" specifies it can be used with 5/16" Grade 70 Transport Chain OR it can be used with 3/8" Grade 43 High Test Chain.  It can be used for both 5/16" or 3/8" chain but they are 2 different grades, with Grade 70 being the stronger of the two grades.  If you use a 5/16" x 3/8" lever binder with 3/8" Grade 70 Transport Chain, the binder will be weaker than the chain, and therefore, becomes the weakest part of the assembly.