This is a checkbox list that helps you to admin many to many relationships in your database. For this to work, you need to specify a mapTableName (in other words, the joining table), a primaryTableName (in other words the table with the primaryKey from which you're working), a primaryKeyValue, and the foreignTableName (the other end of the many to many from the primaryTableName).
If, in Northwind, there was a many to many between Products and Categories with a table called "ProductCategoryMap", and you wanted to use this control from a product's page, you would declare it like this:
<subsonic:ManyManyList id=MrManyMapper runat=server primaryTableName="Products" foreignTableName="Categories" mapTableName="ProductCategoryMap" primaryKeyValue="5">
This would generate a CheckBoxList of all Categories, and would then check off all the categories for the Product with ID 5.
When you want to save the information, you simply call (in code): "MrManyMapper.Save()". This will save all the checked bits to the mapping table, within a transaction.
Again, we only query two columns here - the primaryKey and descriptor for the foreignTable
