Recho,   Those numbers represent the indication rating of an discontinuity. These number may be shown as a negitve or positive number depending on the size or type of discontinuity it is. The Class A thats shown is saying that any discontinuity that has a indication rating of a +5 or lower (+4 +3 +2 +1 -1 -2 etc.) is an rejectable weld. The higher the indication rating is the less detrimental the discontinuity is. Newer UT machines show you the indication rating on the screen and you simply go by which ever table applies to the weld you are checking. If you are using table 6.2, you find a discontinuty that has a rating of +10 then that is acceptable no matter the lenth or location.

Hope that helps.

                                                                                           Travis

Read Clause 6.26...(the whole thing)

Several key things.....
6.26.6.3 adjust the gain so the signal of the indication is at your reference line(50%FSH).
6.26.6.4 figure your attenuation
6.26.6.5 follow the algebraic expression that fits your machine(mostlikey the one for gain)
6.26.7 determine the length using the 6 dB drop method(see 6.31.2 for angle beam)
Now go look at Table 6.2 and see where you indication falls. Don't for get to use the categories listed at the bottom left of the page under that Table6.2(and use the notes)