What does memory_limit = -1 mean in a PHP.ini file?

I was in some ubuntu server configs, and found that my memory on the server (online resources say default is usually 128Mb correct?)

and I found this listed

memory_limit = -1

Why? What does a value of -1 do? Php of 5.6

From the official documantation here:

This sets the maximum amount of memory in bytes that a script is allowed to allocate. This helps prevent poorly written scripts for eating up all available memory on a server. Note that to have no memory limit, set this directive to -1.

memory_limit = -1

Simply means to "have no memory limit" meaning: let the script use whatever is left over from the operating system and other important processes running. So if the machine has 4GIG and the OS + other processes use 2GIG, then your script will get the remaining 2GIG.

From the official docs and the important part is the last sentence!

memory_limit int

This sets the maximum amount of memory in bytes that a script is allowed to allocate. This helps prevent poorly written scripts for eating up all available memory on a server. Note that to have no memory limit, set this directive to -1.

By memory i mean "RAM & Virtual mem"