Pre Botzinger complex is a part of :

Dorsal and ventral groups of respiratory neurons (DRG and VRG) are present in the medulla. However,  lesions of these neurons do not abolish respiratory activity, and they apparently project to the pre- Botzinger pacemaker neurons.​ ​DRG contains mainly I (inspiratory neurons) neurons and is part of the nucleus of the solitary tract  (or nucleus tractus solitarius, NTS).​ The afferent input from arterial chemoreceptors and lung mechanoreceptor synapses on neurons  in the NTS near the DRG. So, major function of DRG is integration of sensory information from respiratory system. ​ VRG is a column of neurons that fire action potentials in phase with respiration. It includes neurons depolarizing during inspiration (I neurons) and expiration (E neurons). All of these respiratory centers are bilaterally symmetrical on the right and left sides of the  brainstem.​ VRG contains three regions that performs specific functions: ​ Rostral VRG: Known as Botzinger complex: Expiratory neurons.​ Intermediate VRG: Pre-Botzinger complex: Inspiratory neurons generate respiratory rhythm. ​ Caudal VRG: Expiratory neurons​. So, pre-Botzinger complex is a part of VRG. Reference: Ganong’s Review of Medical Physiology 25 Ed page no 656​