The best way to remember this is "mills are for bills, and bonds are for buildings". Back in 2015, the voters in 27J passed a bond to help ease overcrowding, but those funds can't be used for things like paying teachers, instructional staff, and upgrading textbooks. "Mills" are the measure used to calculate property taxes in a district. Issue 3D is a narrowly-tailored increase in property taxes to fund things like pay for teachers and instructional staff as well as technology and textbooks.