Sentence:  Although the laws of the various states , in general , specify the situs of property , i. e. , residence or domicile of the owner , or location of the property , the exceptions regarding boats , airplanes , mobile homes , etc. , seem to <WSD>add</WSD> to the uncertainty of the proper origination point for assessment . 
Meaning of the word marked with <WSD> tags:  make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of