Schottky defect, basically a vacancy defect, arises in ionic crystals when equal number of anions and cations are missing from their normal lattice sites, resulting in the formation of a pair of holes.

Schottky defect is generally found in strongly ionic compounds having high coordination number and made up of anions and cations of nearly same size.

On the other hand Frenkel defect arises when a cation leaves it's normal lattice position and occupies an interstitial position.

Frenkel defect is generally found in compounds having low coordination numberand made up of canions and cations of widely different sizes i.e., anion much larger than cation.

Although these conditions seem to be just opposite to each other, yet AgBr shows both the defects. It is because AgBr crystal has a coordination number of 6 which is neither high nor low. The radius ratio of AgBr is also intermediate, thus AgBr satisfies the conditions to exhibit both type of defects.

P.S.: frenkel defect is more commen in AgBr because silver ions due to their small size easily occupy interstitial sites while comparitively larger bromide ions doesn't leave their position that easiy.