In order to ensure that the ratio between the capture of device data and camera data is an integer, data may need to be up-sampled. Nexus will do this automatically during trial reconstruction and will only occur when capture rates are not integers.
If your cameras are capturing at 200 Hz and your device is capturing at 1000 Hz that is not an issue as there are 5 device sub frames per frame of the camera data (eg 1000/200 = 5).
However, if your cameras are capturing at 200 Hz and your device is capturing at 1500 Hz as there are not an even number of sub frames for the device within the 200 Hz camera data (eg 1500/200 = 7.5). In this case Nexus will automatically up-sample to 1600 Hz which is the equivalent of 8 sub frames per frame (eg. 1600/200 = 8). In order to stop up-sampling in this case you may want to increase/decrease the capture rate of the device or the cameras.
It is important to take the capture rate of all devices into consideration when setting up your system for a capture. If you have multiple devices for example if you were to have a 200 Hz camera system, one device capturing at 1000 Hz and another at 1500 Hz, BOTH devices would be up sampled to 1600 Hz upon saving the file.