The bacteriophages used for cloning are the λ phage and M13 phage.  There is an upper limit on the amount of DNA that can be packed into a phage (a maximum of 53 kb), therefore to allow foreign DNA to be inserted into phage DNA, phage cloning vectors may need to have some non-essential genes deleted, for example the genes for lysogeny since using phage λ as a cloning vector involves only the lytic cycle.  There are two kinds of λ phage vectors - insertion vector and replacement vector.  Insertion vectors contain a unique cleavage site whereby foreign DNA with size of 5–11 kb may be inserted.   In replacement vectors, the cleavage sites flank a region containing genes not essential for the lytic cycle, and this region may be deleted and replaced by the DNA insert in the cloning process, and a larger sized DNA of 8–24 kb may be inserted.
There is also a lower size limit for DNA that can be packed into a phage, and vector DNA that is too small cannot be properly packaged into the phage.  This property can be used for selection - vector without insert may be too small, therefore only vectors with insert may be selected for propagation.