Variable or "Get" nodes are a way to externally modify a value for use inside Functions. Unlike Constants, they can be modified externally.
What a Get node essentially does, is grab a dynamic Variable, and return it from the Get Nodes' output for use in a function. These Get nodes form the link between the Input Parameters defined in the Graph Properties, and the Functions being used inside Parameters.
Every time you use a Get node, you must pick an available value from the dropdown menu. Get nodes will grab a value of the corresponding type. That means you will only see valid options in the menu of a Get node, you can never pick an invalid option. If a variable is not available, it means there's a type mismatch
There are a number of "System" Variables: pre-defined special variables that you can not declare yourself. These are quite important, and for the nodes below it is listed what System variables are available.
When a Parameter is exposed, the Function internally consists of only a Get node of the correct type.
Get Integer (1)
$Tiling: Returns the Node current tiling mode. 0: Relative to input, 1: Relative to Parent, 2: Absolute
stampCycleCount: Only used in Substance Painter with Dynamic Strokes.
$time: This variable returns the time in seconds since the Substance rendering engine was started, it can be used to animate Substances in Substance Player.
$depth (FXmap only): Returns the octave (level) number of the FX-Map node. Allows behaviour according to level in the quad-tree.
$depthpow (FXmap only): As above, but returns 2 raised to the power of the octave. This is a helper value for certain common cases.
$size: Returns the size of the current node (in pixels). Does not correspond to UI values in Base Parameters.
$sizelog2: As above, but returns the size as power-of-2 values (ex: for 2048*2048 image, $sizelog2 returns 11). Corresponds to UI values.