It's a preprocessor conditional directive of the compiler. CF means CompactFramework. At compilation time, it is used to know whether or not use some code.
If you aren't using CompactFramework, the next line (_using System.Runtime.Serialization;_) will be evaluated.
However, you only should be aware of user code sections like:
// __LLBLGENPRO_USER_CODE_REGION_START CustomEntityCode
// __LLBLGENPRO_USER_CODE_REGION_END