Thank the gods that we've got people like jester....
But for the benefit of others, direct quotations will suffice I suppose.
***note: all quotations refer to Codex: Orks, 4th Edition, unless otherwise noted in the following***
"USING A FORCE ORGANIZATION CHART
The army lists are used in conjunction with a force organisation [yes, the codex has a typo in it] chart from a scenario. Each chart is split into five categories that correspond to the sections in the army list, and each category has one or more boxes. Each box indicates that you may make one choice from that section of the army list, while a dark-toned box indicates a compulsory selection. We've included the chart used for Standard Missions [and, what do you know?!?! the chart for the upcoming tournament, derp] below" (page 94).
(Image adapted from page 94, immediately following above cited passage for the benefit of the reader) ***IP is property of Games Workshop in its entirety!!!***
___________________
Also, rather than type out the unit entry, I assumed a copy of it would be more helpful
(Image taken from page 102) ***IP is property of Games Workshop in its entirety!!!***
___________________
And, finally, if some players have somehow misconstrued the Big Mek rule titled' Da Big Mek', listed on page 97 of your Ork codices, if these players should have access to one, allows one Deff Dread to be taken as a Troops choice (as the learned jester has already accurately mentioned).
I think from now on I'll need your page numbers for special rules and army list questions.
Bibliography of sources utilized:
1. Kelly, Phil. Codex: Orks. 4th ed. Nottongham, UK: Games Workshop, 2008. Print.