MIFARE 1K classic was designed to have the manufacturer block - block 0 of sector 0, to be non-editable. This block is with card UID data, and factory data. It is originally expected to have each MIFARE 1K card to be unique, which improves the safety of the MIFARE card platform.
But in practice, there are many end users who can lost they keys, the demand of clone cards has been increasing constantly. Then there are some chip companies developed the clone model which leave the block 0 of sector 0 blank, and allow users to input any content as they wish. The MIFARE clone cards was born in this background. And it has become more and more popular especially in locksmith industrial. In one aspect, it provides a lot convenience for some end users, in another aspect, it lower down the safetly level of MIFARE 1K classic cards, as there can be copy ones easily.