If the main line is -6.5 at -110, the alt ladder might offer -3.5 at -175 (safer, expensive) and -9.5 at +140 (riskier, paid for it). Alt-lines are the raw material of teasers and middles, and comparing a book's alt ladder against fair win probabilities at each number is a standard prop-style +EV technique.
Alternate spreads and totals appear as their own market keys in the catalog at /v1/meta/markets, with each rung carrying its point value and price. Because every book's ladder arrives in the same response, cross-book comparison at a specific number - not just at the main line - is a single filter.