Those are called RSBB units. They have holes in the bottom and are designed to be used with vertical and horizontal rebar. If that is all you have instead of solid bottom BB, then you can use a product like grout stop or even roofing shingles laid over the cores that you do not want to fill. The horizontal rebar is located where the knockout is, centered or doubled, as the case may be.
Rebar Channel
I am using this type of block and need to create a channel for horizontal rebar. What tools and method could I use to create a channel similar to the one pictured? Thanks, Jim.
