“Scale” is the minimum a WGA signatory can pay a screenwriter — whether it’s purchasing a spec script or hiring a writer to work on a script.
Each type of job (e.g original screenplay, outline, rewrite) has a set price. These rates are the subject of WGA negotiations, and change every year. In features, rates may be tiered depending on the budget of the film. In television, rates for cable shows will be lower than broadcast shows.
The current WGA schedule of minimums can be found at the WGA’s site here.