It represents the boundary between what is considered to be an ionic or a covalent bond.
Group 1 or group 2 elements with group 16 or group 17 elements. In general, if the electronegativity difference between the two atoms is more than 1.7 in Pauling's scale, then the two atoms form ionic bond.
the difference between 17 and under 17 years old is that under means younger than 17 and 17 is just 17.
17...
13
39
17.
17
4
17.
31
28
23 is.