It is only one of a number of things to look at but it says what is the error of phases. 1 would imply no error (i.e. cos(0)=1 30deg error is cos(30)=0.866)
I really thankful to your concern about my question. Is it a cosine function which you have mentioned ? ( if I am not wrong Cos(30)= 0.154 but you mentioned here 0.86, so does it mean that there is 30 degree error will have figure of merit = 0.154). Could you please elaborate a bit so that I will be more confident about this.