Hi, for the 1920 census, the notation may indicate a new family within a same dwelling. I found this statement on a website for a 1920 census data project: "head numbers can indicate new dwellings and new families. "Head1" usually indicates a new dwelling while other head numbers, like 2+, 2, 3, 4, etc. usually indicate new families within a dwelling".
You might want to check the address of your family member on the census to see if other families share that same address. Hope this is of help. joan