General term for what is left over, often used for quantities, parts, or amounts.
予算の残部は来月に繰り越される。
The remainder of the budget will be carried over to next month.
remaining copies; copies still in stock
Specifically refers to unsold or undistributed copies of books, magazines, or other printed materials.
この雑誌の残部はあとわずかです。
Only a few copies of this magazine are left.