Am I correct in thinking ,initially these yardages are ...

Short 0-2yds
Medium 3-10yds
Long 11 yds plus

Thought I read it somewhere on the site,now I`m not sure.

Can someone please clarify,

Thanks.

Here's a thread that describes it:

http://mfn2.myfootballnow.com/community/3/170

Thanks JDB

Just had another thought JDB,

It works this way to on Defense, the same way as on Offence?

Yes, the both the offense and the defense use the offense's result of this calculation - so if the offense considers itself to be in a short yardage situation, the defense does as well.