Meaning of curly Braces in Manpages

Command syntax is given in a form that is a bit like Backus-Naur notation, described here.

The braces group together two or more options, one of which must be specified. In the case of -I { keyword [,...] | ALL }, that means you either specify keyword [,...] or ALL.

Square braces [...] mean that their content is optional and can either be added to the command or not.

Curly brackets containing pipe-separated items { ... | ... } mean that you must specify one of those items.

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Example:

my_command [--optional-argument] { --either-this | --or-that }

Given the syntax above, you have those options to call the command:

my_command --either-this
my_command --or-that
my_command --optional-argument --either-this
my_command --optional-argument --or-that

Taken from this handy guide,

Some options will have a limited list of choices. A list of choices will be comma seperated and put between braces.

{choice1,choice2} {yes,no}

Where you have -P { cpu [,...] | ON | ALL }, it means you must choose one of the following options. The [,...] part means that you can supply a comma separated list.