Field help (BUDDES) 

If the destination budget exists, it must have the same structure "Accounts / Dimensions" as the original budget:

If the destination budget does not exist, its creation is submitted. This new budget is by default strictly identical to the original budget.

The destination budget can show an amount (or a quantity) that is similar to the original budget but it can also show an increase or a decrease with respect to the original budget, via the coef field.

Examples :
Coef entered = 0. 95 means a 5% decrease.
Coef entered = 1. 20 means a 20% increase.