Auto Select Tool

Auto Select Tool helps selecting pixels that are "similar" by criterias, such as: color, and tone. Clicking on a canvas pixel will automatically select other pixels that are within range, as specified in Tool Options menu.
hotkey: W

Modifier Keys
Ctrl - hold to Add to an existing selection.
Alt - hold to Subtract from an existing selection.

Tool Options
All Layers - tool will sample any visible pixel as it appears in canvas view. Otherwise, only current layer will be sampled.
Contiguous - tool will select pixels that are directly adjacent/chained together, within clicked region. Otherwise, all matching canvas pixels will be selected.
Color - tells the tool how different color code, across Red Green Blue channels, can be for it to still be counted as similar. Number represents total COMBINED difference taken between each channel.
note: for Fill Tool, color difference is evaluated per channel, not combined.
More on Fill Tool here
Tone - is a calculated lightness value of a pixel.

Using Fill Tool - since Fill Tool has a bit different approach to determining which pixels to be affected, it may be beneficial in some cases to use Fill Tool in spree mode, to fill region, then convert spree to selection.

Selection continued...
More information on Selection feature here