# What Are CFX Tags?

CFX tags are custom tags written against the ColdFusion Application Programming Interface. Generally, you create a CFX if you want to do something that's not possible in CFML, or if you want to improve performance of a task in CFML that's repetitive. Unlike CFML custom tags, CFXs are implemented as DLL files and can:

- Handle any number of custom attributes.
- Use and manipulate ColdFusion queries for custom formatting.
- Generate ColdFusion queries for interfacing with non-ODBC based information sources.
- Dynamically generate HTML to be returned to the client.
- Set variables within the ColdFusion application page from which they are called.
- Throw exceptions that result in standard ColdFusion error messages.

You can build CFXs using C++ or Java. Then, to be able to use the CFX, you have to register it in the ColdFusion Administrator.