I was just wondering the same thing because of another thread on the uncapped year. I seem to recall it being referred to as "the final 8", which would lead me to believe it is the last eight teams in the playoffs - and not the eight division winners or teams with the eight best regular season records.
It is the final eight teams in the playoffs. From the CBA:
Section 2. Top Four Teams: Each of the four Clubs that participated in the NFC and AFC Championship games the Prior League Year shall not be permitted to negotiate and sign any Unrestricted Free Agent to a
Player Contract, except: (a) any Unrestricted Free Agent who acquired that status as a result of the NFL waiver system; (b) any Unrestricted Free Agent who was under contract to such Club on the last date of the
last League Year of the player’s most recent Player Contract; and (c) any Unrestricted Free Agent signed
pursuant to Section 4 below.
Section 3. Next Four Teams: Each of the four playoff Clubs that lost in the immediately preceding playoff games to the four Clubs that participated in the NFC and AFC Championship games the Prior League Year shall not be permitted to negotiate and sign any Unrestricted Free Agent to a Player contract, except: (a)
any Unrestricted Free Agent who acquired that status as a result of the NFL waiver system; (b) any
Unrestricted Free Agent who was under contract to such Club on the last date of the last League Year of the
player’s most recent Player contract; (c) any Unrestricted Free Agent signed pursuant to Section 4 below;
and (d) any Unrestricted Free Agent as follows:
(i) One such player for a Player Contract that has a first year Salary of $4,925,000 or more;
and
(ii) Any number of such players for a Player Contract that has a first year Salary of no more
than $3,275,000 and an annual increase in any future contract years of no more than 30% of the first
contract year Salary, not including any amount attributed to any signing bonus. In addition, each such Club
and each such player entering into a Player Contract pursuant to this Subsection may not renegotiate to
increase the amount of Salary to be paid during the term of the Player Contract for a period of one year
after the signing date of such contract.