SSIS Custom Transforms - what is MappedColumnID

SSIS developer documentation is notoriously terse, and I've scoured Google looking for an answer on this...

What is the MappedColumnID property (available on input, output and external metadata columns) all about? It doesn't appear to do anything use开发者_如何学Goful.

Perhaps to rephrase the question... how do I achieve mappings between meta data columns and input / output columns? I know about the ExternalMetaDataColumnID property... but then how/where does MappedColumnID come into play?

Moreover, components like the OLEDB destination have a column mapping editor which appears in both the component UI as well as in the advanced editor. How is this achieved?

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MappedColumnID is just an alternative way of identifying the columns instead of using there names. From MSDN

The use of these properties is not required. These properties provide an easier way for developers to associate related columns, such as input and output columns, in custom data flow components.