The Outlines command outlines the pixels that are attributed to individual objects in the image.
Before any measurement is made within these boundaries, the light must be de-blended, or assigned to one object or another. De-blending allows for a more accurate calculation of photometric measurements such as the computation of the model magnitudes.
Examine the three images below. The image at the far right has the photometric objects identifier selected along with outlines. You can see that deciding what light was contributed by each object presents a challenge for SDSS programmers. The computer must be “instructed” to make a decision about where the edge of the object really is. Checking the Outlines box under Advanced Options displays the edge of the object as the computer “sees” it before the de-blending process is executed. Photometric measurements take place only after de-blending.