Office 2013 licensing - Is a license required on a per install basis or a per user basis?

Hello,

I hope that you will be able to provide me with a clear answers on this as I did not find one on the web.

Short question:

Does Office 2013 require a license on per installed instance basis or on per access basis?

Background:

We are interested in creating an App-Layer that would also include the Office suite. We would like to deploy that App-Layer on 300 client machines however we only have 230 Office 2013 (Standard & Professional Plus licenses).

Is it OK from a licensing perspective to deploy the App-Layer (containing the Office suite) and set permissions so that only the machines that are licensed can access the Office Applications?

Is there any document that explains this in more detail?