The action line is a composite of secondary markets, which bookmakers offer for a sporting event.
Primary markets are the 1×2 market, the total goals/points/games over or under market or the handicaps market on the outcome of a match.
Usually, the action line is wider for major sporting events rather than for the events less popular among punters. The amount of markets in the action line may reach hundreds.
The companies for professional punters which have the lowest profit margin and the highest odds have narrow action line or do not offer it at all.