verifiable and non-verifiable single check valves

[simon dripman]

hi there. i do have my first question. i have read up on
the seriousness of backflow prevention,i understand most of it
and the different devices used but i dont understand what the difference
between verifiable and non-verifiable when they talk about single check
and double check valves.could someone tell me what this means and what the test points of the verifiable are used for as none of my books tell me this as far as i can see.
the dripman

 

[oldplumber]

buy yourself acopy of the water regs guide and all will be explained in detail,

 

[simon dripman]

i have one mate but i have read it twice but i just cant work out the verifiable bit.

thanks anyway though.