In order to write data to a partition, it must first be formatted. This operation should be executed for those partitions onto which the Linux operating system will be installed.
You should not format partitions with data that you would like to keep, for instance user data directories («/home») and partitions with other operating systems. On the other hand, any partition that you would like to «free up», i. e. delete all data in it, may be formatted.
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