Where Exists() will filter an ongoing load by analysing a specific column in the current row and checking whether the value in that column exists in a named field ( ! )
The single parameter version of Exists() will check whether the value in column FieldName is present in the field with the same name. That field may not exist before starting the current load, for example if you you want to avoid loading duplicates.
The two-parameter version will check the field specified in the first parameter for existence of the value specified in the second parameter. Usually this second parameter consists of a differently named cell in the current row, or of a concatenation of multiple cells in the current row.
Your code won't do anything useful because you are checking for a static value in a field that has been created before and does contain value 'C' for sure. Which lmeans that all rows will be loaded anyway.
