Numbering in chlordiazepoxide
The parent structure of chlordiazepoxide consist of the heteromonocyclic component 2⁠H-1,4-diazepine that is fused to a benzene ring.



According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), the name of the fused parent structure is generated as follows:


  P-25.2.2.4 Heteromonocyclic components fused to a benzene ring
  
  Unless listed as a retained name (…), a benzene ring fused to a heteromonocycle of five or more members (a benzoheterocycle) is named by placing the locant(s) indicating the position(s) of the heteroatom(s) at the front of the name consisting of the fusion prefix ‘benzo’ followed by a retained name, a Hantzsch-Widman systematic name, or a name formed by skeletal replacement (‘a’) nomenclature (…). The locants cited correspond to the full bicyclic structure. As in Hantzsch-Widman names, locants are placed in the order corresponding to the order of citation of the heteroatoms in the heterocyclic component. The locant ‘1’ is always assigned to the atom of the heterocyclic component next to a fusion atom. Heteroatoms are allocated lowest locants as a set, without regard to kind; if there is a choice, lowest locants are assigned in accordance with the seniority of the ‘a’ prefixes (…). In general nomenclature locants may be omitted when the name is unambiguous; for preferred IUPAC names locants must be cited. The letter ‘o’ of the ‘benzo’ prefix is elided when followed by a vowel. Indicated hydrogen is placed at the front of the name, when required.
  (…)


Accordingly, the name ‘benzodiazepine’ consists of the prefix ‘benzo’ followed by the name ‘diazepine’. The locant ‘1’ is assigned to the atom of the heterocyclic component next to a fusion atom and the heteroatoms are allocated lowest locants as a set (here: 1,4): 



The same numbering of locants applies to the complete structure of 7-chloro-2-(methylamino)-5-phenyl-3⁠H-1,4-benzodiazepine 4-oxide: