Footprinting as a term, I did not know of until yesterday. However it does what it says on the tin; depending on your preference. In a computing world it is used in reference to deep stuff such as DNS and other IP data from the ‘system’ http://en.wikipedia.org/wiki/Footprinting
Outside of the tech world footprinting is an audit that provides an assessment of substances, so a footprint could show the carbon emissions that a mobile phone manufacturer is responsible for in a 6-month period across its total organisation, or the carbon impact from the manufacture of a single handset. More and image from http://www.wspenvironmental.com/expertise/footprint-definitions
Why is this interesting to My Digital Footprint – more data. As I expand the data input model from just your digital data, footprinting (outside tech) could give you more information on the consequences of what you have done – that does become interesting.