There is an entire chapter in the PBI book on uniformity, along with a volume of case law and Article VIII, Sect 1 of the PA Constitution, but generally it means all properties in the same category must be assessed similarly. For instance, if your assessed fair market value for a 1 acre lot with a medium-size house is one value but a similar neighbor's house is noticeably less, then there is a uniformity issue. Same class, same assessed value.