Hi Miguel
If you want to pass a piece of content (html) to a function, you can either:
1) define a parameter define a parameter of type XhmltDocument.
2) define a string parameter, and set parameters widget to be VisualXhtmlEditor
Alternatively, you can define custom content in data types (equivalent of a stand alone sql table), page folders (data associated with a page) or page meta data. In this case function either takes data associated to the current page, or you can pass a data reference to a specific item/record as a parameter value.
More information on working with custom data in C1 you can find here http://docs.composite.net/Data