Base tax value
The base tax value (abbreviated BTV) of a province or city is a number upon which province tax is computed.
Province Base Tax Value
In a uncolonized province, the base tax value is the supply limit for army attrition. Other than that it has no effect in the game. However, you need to know which provinces have a high base tax in order to colonize them first. To see the province base tax value, just select the province. The left number on the province screen is the base tax value.
The base tax value of provinces is fixed in province.csv, and cannot change.
Settlement Base Tax Value
When a colony or trading post is created, it takes the base tax value of the province it is created in. The base tax value does not have any monetary effect in a colony or trading post -- they don't collect most taxes. But it does serve as the base of the supply limit computation.
Changes to Base Tax Value
The base tax value of cities may change, but only via event. There are scripted events for many major powers giving them boosts to base tax value. In addition, there are several random events which can change base tax values for cities of any country. Examples include New Land Claimed, which increases base tax by 1 in a city. The neutral event Sale of Offices gives you a choice of losing one point of base tax in a city to gain 100 ducats. The Indulgence Peddler event is an example of a nasty event lowering base tax value.