Pin Stress-Relief Groove (SRG) and Box Boreback (BB) fall under Product Optional Features in API 7-2 Specification for Threading and Gauging of Rotary Shouldered Thread Connection. These are features designed to help reduce the likelihood of fatigue cracking in the highly stressed area by removing a certain length of the unengaged threads to reduce stress concentration. 

Stress-relief grooves are not recommended for use on pin threads with pitch diameter, C, at gauge point smaller than 89 mm (3.5 in). Do note, SRG on pins cause a slight reduction in the tensile strength and section modulus of the connection. However, under most conditions, this reduction in cross-sectional area is more than offset by the reduction in fatigue failures.

The boreback feature is not recommended for use on threads with pin length, L_PC, smaller than 89 mm (3.5 in).  

SRG and BB are not required on Drill Pipe, and is optional for Heavy Weight Drill Pipe and Drill Collar under API Specification and DS-1^® Standard level. It is however required for Heavy Weight Drill Pipe and Drill Collar under NS-1™ standard and DS-1^® Critical level.

SRG and BB are also not applicable for Double-Shouldered Connections as it interferes with the secondary shoulder. Introducing the BB in the box connection, the secondary shoulder which contributes to the increased torsional strength will be removed.