php ini zlib.output_compression Level

zlib.output_compression_level is used to transparent output compression. This Specify a value between 0 to 9 compression. The default value, -1, lets the server decide which level to use.

; zlib.output_compression_level = -1

php manual there is this zlib.output_compression_level that defines the level of the compression and the default value is -1. yet the manual did not specify anywhere how does this value affect the compression.