What is Address Input in avalon UART IP?

[Altera_Forum]

I want to use UART IP in Qsys. While going through the signal discription I found 3 bit address input. What is it?  

Is there any document that I can refer to get the details of the same?   http://www.alteraforum.com/forum/attachment.php?attachmentid=10667&stc=1

[Altera_Forum]

The address is referring to register map in here pg7-11 https://www.altera.com/en_us/pdfs/literature/ug/ug_embedded_ip.pdf