An alternate line is a spread or total that is a derivative of the standard betting market. These lines will be higher or lower than the main line being offered at (or around) -110 odds. Alternative lines that have a higher win probability will have shorter odds. Vice-versa, alternative lines with a lower chance of winning will have longer odds.
An example of an Alternate Line is an Alternate Total for Match Score which offers a higher or lower total with odds changing accordingly (such as in NFL a Total being 50.5 -110 and an Alternate Total being 49.5 -167). It can also be used in spread/line betting in the form of an Alternate Handicap which offers a higher or lower spread/line (such as in NBA a team being +4.5 -110 and an Alternate Handicap being +5.5 -167).