User MRU

Hello,

I'm trying to find out more information about User MRU and specifically MRU's for Active Directory Users. 

Were these introduced in Office 2013?

Also

Do you know how the unique ID for each Active Directory user is created? e.g.  -AD_FDCD3F40D608189D8DEE5E446CA139BA6C1C9F56F9D336E729FC432249798A69

And

Weare using Windows 7 and are noticing that User MRUs are created for some users and not for others? Is anyone aware of MRU problems on Windows 7?

Thanks in advance