Condo qualifier
A code added after the block and lot to tell apart units that share the same parcel. For a condo unit it is the letter C followed by the unit number, such as C0001.
New Jersey example
Say a condo building sits on Block 1, Lot 1. Every unit shares that block and lot, so each gets its own qualifier. Unit 1 could be listed as Block 1, Lot 1, C0001, with leading zeros. When you look up a condo's assessment, include the qualifier, because the block and lot alone match every unit. Other qualifiers include QFARM for qualified farmland.
Official source
MOD IV User Manual (NJ Division of Taxation, Property Administration, 2024 updates)
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.