A given file might be part of a Snapshot copy. The changes to such a file are written to new blocks. Therefore, the blocks within the Snapshot copy and the new (changed or added) blocks both use space within the volume.
Changing the contents of the myfile.txt file creates a situation where the new data written to myfile.txt cannot be stored in the same disk blocks as the current contents because the Snapshot copy is using those disk blocks to store the old version of myfile.txt. Instead, the new data is written to new disk blocks. As the following illustration shows, there are now two separate copies of myfile.txt on disk—a new copy in the active file system and an old one in the Snapshot copy: