How can eval code not have a calling context?

[Mark S. Miller]

ES3 says:

10.2.2	Eval Code
When control enters an execution context for eval code, the previous
active execution context, referred to as the calling context, is used
to determine the scope chain, the variable object, and the this value.
If there is no calling context, then initialising the scope chain,
variable instantiation, and determination of the this value are
performed just as for global code.

I am baffled by "If there is no calling context,". How could the
possibility arise? How would eval get called if no one calls it?

-- 
    Cheers,
    --MarkM