Internal help for this application in Asterisk 1.4: 

  -= Info about application 'Pickup' =- 

[Synopsis]
Directed Call Pickup

[Description]
  Pickup(extension[@context][&extension2@context...]): This application 
can pickup any ringing channel
that is calling the specified extension. If no context is specified, the
 current
context will be used. If you use the special string "PICKUPMARK" for the
 context parameter, for example
10@PICKUPMARK, this application tries to find a channel which has define
d a channel variable with the same content
as "extension".

diff output to internal help in Asterisk 1.2: 

8c8
<   Pickup(extension[@context][&extension2@context...]): This applicatio
n can pickup any ringing channel
---
>   Pickup(extension[@context]): This application can pickup any ringing
 channel
10,12c10,11
< context will be used. If you use the special string "PICKUPMARK" for t
he context parameter, for example
< 10@PICKUPMARK, this application tries to find a channel which has defi
ned a channel variable with the same content
< as "extension".
---
> context will be used.
>