Entry Method
Some tools cannot cut "downwards", meaning in order to change the depth one has to lower the tool in an angle. This can be either straight along the actual path or in the form of a helix that spirals down to the desired depth and then follows the path again.
Holding Tags
Holding tags are just that, little bridges left when cutting out a part in order to hold it in place and prevent it from flying right in my face (and getting wrecked in the process of doing so). While they could have any shape the simplest forms are rectangular, trapezoidal and triangular.
Dressups
Now that I think about it it would be great to have an icon for dressups in general. A dressup is a modification of an existing path. Of the above dressups "Entry Method" is probably the only that makes sense for any path. The other 3 are really only useful for Contour and Profile.
There are the 4 dressups that currently don't have an icon (and to be clear, this is for 0.17) - in alphabetic order:
Dogbone Dogbones and T-Bones are little extra cutouts one has to make if a corner needs to fit another part. A concise and graphical description can be found here: http://blog.inventables.com/2014/06/learn-about-milling-inside-corner.html
Dragknife A dragknife is a special tool that requires modifying the path. The best description is really sliptonic's video: https://www.youtube.com/watch?v=40CRimcwAZo
Entry Method Some tools cannot cut "downwards", meaning in order to change the depth one has to lower the tool in an angle. This can be either straight along the actual path or in the form of a helix that spirals down to the desired depth and then follows the path again.
Holding Tags Holding tags are just that, little bridges left when cutting out a part in order to hold it in place and prevent it from flying right in my face (and getting wrecked in the process of doing so). While they could have any shape the simplest forms are rectangular, trapezoidal and triangular.
Dressups Now that I think about it it would be great to have an icon for dressups in general. A dressup is a modification of an existing path. Of the above dressups "Entry Method" is probably the only that makes sense for any path. The other 3 are really only useful for Contour and Profile.
Summary of Icons Needed