When VAT is raised, prices rise and consumption falls somewhat, so revenue grows less than the static estimate.
Of every 100 euros that a change to “General VAT rate (now 25.5%)” saves or brings in, 10 € is lost as revenue on the line “Value added tax” every year. (The other way round if the change adds spending or cuts taxes.)
E(t) = s × T(t)
Bounds: if households keep the euro amount of their spending unchanged and the increase passes fully into prices, the tax base shrinks and 1 / (1 + VAT rate) ≈ 80% of the static revenue is collected, a loss of about 20%. If some spending is financed from savings or the increase does not fully pass into prices, the loss is smaller. The default of 10% is in between; it is an estimate.
How the effect builds up over time
Share of the change's own effect on the balance, year by year while the change continues: −15% means 15% of a saving comes back as cost (or 15% of extra revenue is lost). With the default values.
You can switch the effect on or off and adjust its assumptions in the simulator under Cause-and-effect chains.