Final answer:
Custom timestamp extraction is usually specified in data parsing, used in programming languages and logging frameworks to define a pattern for datetime data manipulation.
Step-by-step explanation:
Custom timestamp extraction is typically specified in the context of data parsing processes, especially when dealing with logs or data stream management systems. In programming, this might require specifying a pattern or format for the datetime data that is being manipulated or displayed. Many programming languages and logging frameworks allow you to define a pattern that matches the timestamps in your data, facilitating their extraction and manipulation.