The mode describes the behavior of zlibc in certain
situations. These are:
What to do when a file is accessed in readonly mode.
What to do when a user program tries to append data to an existing file.
What to do when a user program tries to create a new file.
What to do when a user program tries to access an existing file and none
of the preceding situations apply (see section Other write requests).
Whether to show the size of the uncompressed file or the size of the
compressed file as a response to stat (see section Size shown for compressed files).
The mode for each off these 5 situations has to be described separately.
For each of these 5 situations, the scan through the class description
is done until the reaction of zlibc for this situation is defined.