[jsmith45]

That is a Brita Stream product.

For the pitcher equivalent they document that the indicator is simply a count of the number of pours, where they assume each one is exactly 8 oz, and that the filter lasts for exactly 40 gallons. (i.e. turns red after exactly 640 pours).

For that one, they don't actually document how it works. They do seems to indicate they that they are trying to approximate water used though, since they say it turns red "once you reach 40 gallon[s]". It might be a count of activations of the spigot handle or based on timing how long the spigot handle has been open since last reset (e.g. they previously calculated the flow per second, and use that to count up seconds that up until they reach 40 gallons).