difference between fractional and relative bandwidth

Good morning,

i have problem with few definitions, and i will be so grateful if someone can help me,

i really want to know:

1. what is the difference between the fractional and relative bandwith ? (some books said there is difference and some books said that they are the same)
2. why we mesure the bandwidth at -3 dB?
3. why we mesure the UWB bandwith at -10 dB?

my respects,

1-I think they are not same. Fractional Bandwidth is a portion of a bandwidth that is constant, Relative Bandwidth is a percentage of a bandwidth that is variable.I have extracted this result by definition..
2- -3dB is a point so that the power is halved. e.g. 10log(P/2)=10log(P)-10log(0.5)=10log(P)-3

thank you so much for the helpful answer

- if you allow, can you please telle me what is the diffenition of fractional and relative bandwith that you have

- and i'm working on UWB (3.1 - 10.6 GHz) bandwidth, and i read definitions about how to calculate and consider this bandwidth as an UWB, and sometimes i find that the relative bandwidth must be upper than 20 %, and sometimes the fractional bandwidth is upper than 20 %, so wich difinition is the most correct?