There is a set of handwritten marks for each family, noted in the head of household's Name field. The notations have two lines, like "6-1 / of 6" The first and last numbers seem to be the number of persons in each household. There is also notations in the Relation field (e.g. 4C), which I think is the number of children in the household.
I remembered that there was a discussion of these marks in my extensive readings. I searched the Ancestry Article Archives and didn't find one that addressed this. I checked the reference books I have available on my bookshelf and didn't see anything.
I Googled [1900 census extraneous marks] today and found the discussion thread on the APG mailing list from June 2009 - Drew Smith's post title 1910 census help...
"You may be thinking of this article:http://www.archives.gov/publications/microfilm-catalogs/census/1920/part-01.html
"It includes the following text:
"The microfilm may also show code numbers or letters in some of the columns. Clerks added these codes in red ink (which cannot be distinguished on the microfilm) after the census, to be punched into the cards used to tabulate the census results. The codes usually represent household composition, occupation and class of worker, or simply marks made in the coding, punching, or tabulating operations themselves and should be ignored."
Linda's example had only the single set of extraneous marks on the census. The census page for my Austin Carringer family in the 1900 census has several more sets of extraneous marks: