Now that Select > Convert to Seamless Pattern is gone, you should know about a special role for selections in the Seamless Tiling filter.

Selections protect an object against being blurred during the tiling process.

Another image shows the effect of the Mirror mode with Offsets both zero and applied with a Width of 100.

The third image shows the same settings but applied to the original in which the tailor's dummy was first selected.

Note that it is the main version of the object that is protected from blurring. It's duplicates used in the blending process are still blurred. You should be able to pick this out in the images I've provided

Balls And Bubbles
Has anyone noticed that in Multiple Balls or Bubbles mode of Balls And Bubbles the image you get tiles seamlessly?

Preserve Lightness works like this:
When you select a target color to replace a source color in the image and Preserve Lightness is unchecked, the source color becomes the target color exactly. This certainly changes the color balance in the image but, unless you have an exceptional eye, it also changes the contrast of the image. To prevent contrast changes you can check Preserve Lightness. When this is checked the target color's lightness is replaced by the source lightness while the hue and saturation of the target color are still used to replace the source. The result is a color change without a contrast change.

Balance To Grey works like this:
When correcting color in an image it is often quite hard to say what the new color of some spot in the image should be. You know the existing color is wrong but it is hard to guess a replacement. However, the eye is usually very sensitive to color tinges in colors that should be completely neutral (i.e. achromatic black, white or grey). Switching on Balance To Grey gives you the convenience of only grey shades and you can click up to three places in the image that you want to be neutral.

When you have both Preserve Lightness and Balance To Grey checked it actually doesn't make any difference which grey swatch you pick since the result lightness of the grey will be whatever lightness was at that spot in the original image. The only change is that the source color becomes completely desaturated.

I am having a difficult time with picking the Gray choice and wonder what the best way might be to choose such a target color to represent as Grey?

There is a "feature" here copied from Photoshop. In PS you must pick the White and Black points first and the Grey point last. If you don't, a previously set Grey point becomes unset when you choose Black or White. This is a major PITA in my view and is being removed at the moment.

By the way, note that you can set the Black point to white and the White point to black to get a negative image - just uncheck Preserve Lightness first. With other color choices quite bizarre - nay, freaky - results are possible, a case where Randomize may be interesting.

One last note. This filter uses from one to three color equivalences (original and desired result) to build a look-up table that converts all the colors in the image. There are certain constraints on the equivalent colors so that a smooth look-up table is obtained. (Exactly the same situation obtains in PS, though we have eliminated some of the errors in the PS implementation.) What this means - especially if you use wild colors much different from the original - is that, once you have defined two points, the third point is sometimes reported as invalid because it doesn't meet the constraints. Unfortunately, it is very difficult to provide guidance as to what would be valid under these circumstances and you might find yourself doing a lot of trial and error. If you do get stuck this way, try redefining your Black point to be a much darker color and your White point to be a much lighter color. Bear in mind that this lightness relationship must be true in all three color channels.

Thanks. I have been just using the Black and White, and love the way that adjusting those two have added some nice change to an image.