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"?" is a help feature which makes Cisco IOS a rich CLI Operating System. So even without knowing the full syntax of a particular command, anyone can figure out the right commands using this help feature.
Although all of above answers are somehow correct. Elaborated further, on any command you can have two kinda help. (i) Detailed help via 'help' keyword while putting it before command and (ii) via putting '?' character which is used only to get command syntax. However, if we enter a '?' after a menu command it behaves like a tab key and provide commands list that a available
It will give you the rest of the options to finish the command.
help to complete a command in case u forgot.... "?" shows what are probable parameter can execute with after given parameter...
in Cisco ios, command-line interface(CLi) a brilliant feature has been in the form of '?' which is a symbol of help is needed, anyways, it shows the next available parameters you can choose from but there is a catch, only a know syntax can be executed otherwise an invalid command prompt will be shown.