Thank You.Well, the IRS doesn't actually have a "discuss everything" wildcard. But fortunately you shouldn't need one. Your interactions with the IRS relate to what you are obliged to file.
So, what are you obliged to file? IRS Form 1040 and its various attachments and schedules is one form. Anything else? IRS Form 3520 and/or 3520-A, perhaps? Most individuals don't have much if anything beyond good old "1040."
If Form 1040 is all you're obliged to file, then as the instructions indicate you put "Income" in 3a. In 3b you would put "1040." The IRS will accept 2008-2018 in 3c, yes -- that complies with the instructions. You can list any year prior to 2018 as the starting year. You can list only up to 2018 as the ending year if you file this form in 2015, because 2018 is 3 years (the maximum) after December 31 of the year when you file the form.
Or, if you want to include 3750 and 3750-A (if applicable), you would put "Income" in 3a, "1040, 3750, 3750-A" in 3b, and your desired years.
Designating someone to discuss your (future) estate tax return is fairly common...and perhaps morbid. To do that you'd list "Estate" in 3a and "1041" in 3b, separately or in combination, as desired. Obviously the (still living) person filing Form 1041 is able to discuss the filing with the IRS. If you filed a 1041 for someone else (a dead person), then I think you can grant discussion rights with the IRS to that 1041 using this form.
As yet another example, IRS Form 8854 is a sometimes discussed form in this forum. You could add that to the list as "Expatriation" (3a) and "8854" (3b) if you wish, if applicable.
If you later discover you forgot something, no problem: just file another one of these forms to fill in the gap(s).
Yes, if you want to grant unlimited discussion authority within the scope of columns a, b, and c, put "Not Applicable" in 3d.
For line 4, I'd just refer you to the instructions since I don't know specifically why you're filing Form 8821, but usually you would leave that box unchecked, yes.