Whenever the Project ROI is specified, such that it can be projected in the image space - that is, either a bounded volume (2D limitation + Z range), or a 2D region coupled with a specified terrain height - a prior selection of the input images that capture it is possible.
In the GUI, the user will be queried during Project Definition wizard, whether this selection should be performed. Enabling it can potentially accelerate the processing significantly, so this is recommended.
For the CLI, the flags bellow enable/disable this option. Note that the -initterrain flag enables it by default. If these flags are not passed at all and the Project ROI can be mapped in the image space, the image selection is enabled by default.