The study area in which you are interested may span several image files. In this case, it is necessary to combine the images to create one large file. This can be done by using the Mosaicing process which is explained in details in this course. On the other hand, the course also explains the subseting process which is used when an image file contains areas much larger than a particular study area. The subset process reduces the size of the image by cutting the area of interest in a new image file.