In the 15w32b snapshot, a new tag parameter was added to the /scoreboard players command. However, the syntax was not updated. It shows the following:

/scoreboard players <set|add|list|remove|reset|list|enable|test|operation> ...

While it should actually include the tag parameter, like so:

/scoreboard players <set|add|list|remove|reset|list|enable|test|operation|tag> ...

Note: That's pretty much the exact same thing that happened with the missing add parameter in the /worldborder syntax (see MC-58908). It keeps happening!