By default, the system time will be used as the time reference for the timestamp applied to each recorded video frame. The timestamp is applied immediately when the frame header is received by the software.  This means the applied timestamp is relative to the start of the frame transfer.  In multi-camera configurations, timestamping frames relative to the start of exposure is often preferred over timestamping relative to the start of frame transfer, especially if the cameras have widely-varying exposure times. 

Most USB 3.0 Vision cameras provide timestamps in the header of each image. These timestamps are generated by the camera as the frame is exposed and are usually relative to the start of the exposure period.  To timestamp frames based on this camera timestamp instead of the start of frame transfer, check the "Use camera timestamp" setting on the camera's "Connect" tab.

Note: The camera's internal timestamp clock is synchronized with the system time (or another reference) at the start of acquisition. As the recording progresses over a long period of time, this internal time will drift relative to the reference time. Even two identical cameras will drift relative to each other due to variances in their internal time counters. The result is that the timestamps near the end of a long recording can be noticeably different than the reference time.

Timestamping frames relative to the start of frame transfer is the default and recommended setting as it uses a common time reference, avoiding the potential for timestamp drift between multiple cameras.