Allowances are often used as a "plug #" to cover an anticipated cost for material selections that have not been finalized.
A house flipper might know they want to replace the vanities in the house, but they aren't sure which specific vanities they are going to use so they just plug in a $750 "allowance" for the vanities which they know is adequate to buy a reasonably priced vanity for the project.
The Materials List Report automatically identifies the "allowances" on your project if there has not been a material selected for the repair item.