The CSS property this relates to is
It doesn't display it within Sketch's interface, but the value is in pixels. So for your example of
0.2, that would translate to
If you right click on any object within Sketch, you can
Copy CSS Attributes and it'll grab as many attributes and values that particular object may have.
So in the context of a Text layer, this is the raw output:
/* Roundup of the lates: */
Lastly, here's documentation of the spec:
And what the value actually does (emphasis mine):
Specifies extra inter-character space in addition to the default space between characters. Values may be negative, but there may be implementation-specific limits. User agents may not further increase or decrease the inter-character space in order to justify text.