SVG $ position?

[ADKDeno]

I am fairly new to SVG, so this may be a trivial question.  

The clock face tutorial uses a $ in the x position:

https://dev.fitbit.com/guides/clockfaces/

I've searched through several SVG guides and can't find what the $ represents?  Can someone point me at a resource that explains what the $ means?

[SunsetRunner · Accepted solution]

The $ is a special value which is always equal to the current value of the parent element. In the Clock Faces guide, it's the x position of the g element.

You can find more information on the documentation.