Here is the Gimp Tutorial page for the Ellipse Select Tool, we will go in to more details for those features highlighted in green above.

Highlight

If you enable this option, the selected area is emphasized by a surrounding mask to make visual selection much easier.

This can be seen in the image below:

Guides
Guides are in the drop down menu as shown highlighted in yellow below:


With this menu you can select the type of guides that is shown within the selection to make the creation of a selection easier, respecting Photo composition rules.

No Guides - No guides are shown at all.

Center lines - Only center lines are shown as guides within the selected area.

Rule of thirds - Guides are shown along the rule of thirds while creating the selection

Golden sections - With this option, guides show up to mark the golden sections.

In the next frames we will show what these guides look like and what some of these Photo composition rules mean.

(1) No Guides
(2) Center lines
(3) Rule of thirds
(4) Golden Sections

Guides # 1 & 2 are self explanatory. Guide #3 is the "Rules of thirds" that most photographer have learn when composing their shots. Here is an explanation and the page source: view link

Golden Section The golden section is a certain length that is divided in such a way that the ratio of the longer part to the whole is the same as the ratio of the shorter part to the longer part.

Say what!!!! LOL

Forget that, try this instead:

Golden Section Rule

It has been found that certain points in a picture's composition automatically attract the viewer's attention. Similarly, many natural or man-made objects and scenes with certain proportions (whether by chance or by design) automatically please us. Leonardo da Vinci investigated the principle that underlies our notions of beauty and harmony and called it the Golden Section. Long before Leonardo, however, Babylonian, Egyptian, and ancient Greek masters also applied the Golden Section proportion in architecture and art.

To get a clearer sense of these special "Golden" composition points, imagine a picture divided into nine unequal parts with four lines. Each line is drawn so that the width of the resulting small part of the image relates to that of the big part exactly as the width of the whole image relates to the width of the big part. Points where the lines intersect are the "golden" points of the picture:
So what did we learn here? We learn that we can make a more pleasing image by using the "Golden Sections rule and here is the best part.....the selection tool does the math for us! Yes!

Forgot to mention that in the image above you will see the two towers and the people all intersect with the golden section lines. This is what it means to compose an image with these rules. After you have taken a photo you may be able to do this by cropping your image. It is better to mentally do this when taking the picture because cropping won't always get the right results. If the people were standing 30 feet farther to the left then cropping would not help. But if you moved over far to your left you may be able to get the points of interest within the golden sections with a different angle Enough on that, let's figure out what this "Shrink" thing is!.

Auto Shrink Selection The Auto Shrink Selection check-box will make your next selection automatically shrink to the nearest rectangular shape available on the image layer. The algorithm for finding the best rectangle to shrink to is “intelligent”, which in this case means that it sometimes does surprisingly sophisticated things, and sometimes does surprisingly strange things. In any case, if the region that you want to select has a solid-colored surround, auto-shrinking will always pick it out correctly. Note that the resulting selection does not need to have the same shape as the one you sweep out.

Hmmm...let me play with this....alright, I am back! First thing I noticed was that my "Auto Shrink" button was not highlighted so I could not click on it. I found out through play that first you must select something on your image! Then the button is active. So basically in boomer English this means. You select something, hit the "auto shrink" button and it will produce a selection similar to the nearest shape on the image.

Shrink merged
If Sample Merged is also enabled, then Auto Shrink will use the pixel information from the visible display of the image, rather than just from the active layer. For further information regarding Sample Merge, see the glossary entry Sample Merge.

Okay here is what I found. Some of the other tools have a "Sample Merge" check box. If any of these is checked then the "Shrink Merge" allows the "Auto Shrink" to shrink the selection according to information on all of the layers, not just the one you are working on. I doubt if I will ever use this but there you are, we are done with the Ellipse Select Tool.

Questions?

Huh???????????..lol