The tool settings contain extended settings for the tools in the toolbar. Most tools have a variable intensity. Sometimes color channels can be locked, e.g. it is possible to edit only the red channel.
If there is an open document, many tools, that have a slider as main control, allow modifying that slider by using the keyboard. Typing only a single digit, uses this digit multiplied by 10 as percent value (i.e. "1" results in 10%), "0" is translated to 100%. Entering a longer number uses this number directly as percent value.