Improvement ratio
The share of a parcel's total assessment allocated to improvements rather than land.
New Jersey example
If improvements are $240,000 of a $300,000 total assessment, the improvement share is 80%; that describes the assessment split, not construction quality or market value.
Official source
https://www.nj.gov/treasury/taxation/lpt/localtax.shtml
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.