p3p is: Platform for Privacy Preferences Project (p3p)
In some instances cookies will not be accepted unless a p3p technical document is provided. The document explains what the cookie is meant to do. Based on security protocols;
- the intention of the cookie and
- user privacy setting,
Based on these security protocols cookies are accepted or rejected.
Typical policy evaluation: Internet Explorer will evaluate the privacy policies that accompany a cookie, and determine whether the cookie will be accepted.
On every person's computer, the privacy level, which the user sets, is used to determine how Internet Explorer will assess the cookie & its acceptability.
Other important variables that Internet Explorer looks at are whether the cookie is a persistent cookie or a session cookie, and whether it is a first party or third party cookie.
How it works / how you can confirm
So, you can only take a look when the cookie is blocked, and generally this happens only when you set your Internet Explorer Privacy Settings to High or Very High.
Note: this whole process is highly technical, and there are numerous lengthy publications on this subject.