On the IPUMS-USA site, it says that HHWT should be used when trying to get a count of subfamilies rather than the perwt for the reference person in that subfamily. However, I am confused as to whether or not the household weight should be used for each subfamily in a household, or if I should only use each hhwt once regardless of how many subfamilies are in the household. For instance, if a household had a weight of 100 and there were 2 subfamilies in the household, would the estimate of the number of subfamilies resulting from this household be 200 or 100?
I agree that this is confusing, but IPUMS-USA recommends using HHWT for subfamily estimates because this is what the Census Bureau uses to generate their estimates. It is related to a weighting adjustment introduced in 2006, briefly described in this paper from the Census Bureau and here is the much more detailed report.
I hope this helps.