It is very common that sometimes when you import content from a file (eg. CSV or XML file) which is provided by someone else, you are not pleased with the format. You might see many inconsistency values which obviously would need some improvements in order to have a clean import.
That is when Feeds Tamper module comes to save your task. It comes with multiple plugins which manipulates values before mapping them. That gives great power for site builders to have clean feeds imports with consistent values.
However, sometimes you will not find your plugin to do something you want and that is why I write you how to write your custom plugin for feeds tamper.