What are the icons on a component's input/output parameter?

Not sure if it qualifies as a FAQ but I wanted to reference it in other threads without having to post the same thing over and over.

I/O Modifiers what are they and what do they do?

The I/O Modifiers can be applied from the right click Context Menu of either a component's input or output parameters. With the exception of <Principal> and <Degrees> they work exactly like their corresponding Grasshopper Component. When a I/O Modifier is applied to a parameter a visual Tag (icon) is displayed. If you hover over a Tag a tool tip will be displayed showing what it is and what it does.

The full list of these Tags:

1) Principal

An input with the Principal Icon is designated the principal input of a component for the purposes of path assignment.

For example:

2) Reverse

The Reverse I/O Modifier will reverse the order of a list (or lists in a multiple path structure)

3) Flatten

The Flatten I/O Modifier will reduce a multi-path tree down to a single list on the {0} path 

4) Graft

The Graft I/O Modifier will create a new branch for each individual item in a list (or lists)

5) Simplify

The Simplify I/O Modifier will remove the overlap shared amongst all branches. [Note that a single branch does not share any overlap with anything else.]

6) Degrees

The Degrees Input Modifier indicates that the numbers received are actually measured in Degrees rather than Radians. Think of it more like a preference setting for each angle input on a Grasshopper Component that state you prefer to work in Degrees. There is no Output option as this is only available on Angle Inputs.

7) Expression

The Expression I/O Modifier allows you change the input value by evaluating an expression such as -x/2 which will have the input and make it negative. If you hover over the Tag a tool tip will be displayed with the expression. Since the release of GH version 0.9.0068 all I/O Expression Modifiers use "x" instead of the nickname of the parameter.

8) Reparameterize

The Reparameterize I/O Modifier will only work on lines, curves and surfaces forcing the domains of all geometry to the [0.0 to 1.0] range.

9) Invert

The Invert Input Modifier works in a similar way to a Not Gate in Boolean Logic negating the input. A good example of when to use this is on [Cull Pattern] where you wish to invert the logic to get the opposite results. There is no Output option as this is only available on Boolean Inputs.