Column is the full-text filter for columns that participate in full-text indexing on a table. (Visual Database Tools)
You are trying to delete a column or change its data type, but that column is the full-text filter for one or more other columns for which a full-text index exists. If you proceed with the operation, the columns will no longer participate in the full-text index.
If you later want to reestablish full-text indexing for these columns, you must choose or create a new full-text filter field, and you must add these columns back to the full-text index. For more information about accomplishing these tasks, see the topics on full-text indexing in SQL Server Books Online.