It's the FCC's "significantly viewed" list, which can be found at
http://transition.fcc.gov/mb/significantviewedstations120611.pdf
Here is the list for Bristol County, MA, which is in the Providence DMA and not Boston.
Bristol
WLNE-TV, 6, Providence, RI (formerly WTEV)
WJAR, 10, Providence, RI
WPRI-TV, 12, Providence, RI
+WNAC-TV, 64, Providence, RI
WBZ-TV, 4, Boston, MA
WCVB-TV, 5, Boston, MA (formerly WHDH)
WHDH-TV, 7, Boston, MA (formerly WNAC)
WSBK-TV, 38, Boston, MA
#WLVI-TV, 56, Cambridge, MA (formerly WKBG)39
Notice how we have every major Boston channel, except for WFXT, on that list. Because of the fact that channels 4, 5, and 7 are on those list, Comcast and FIOS can carry those Boston channels in their entirety across Bristol County, despite us being in the Providence DMA. Also, we get those channels on our HD lineup too, so we have two of every network channel in HD, except for FOX.
Now, WFXT is not on that list. Because of that, cable providers in Bristol County, MA, can carry WFXT, but they have to blackout the duplicating network programming (like American Idol) on WFXT. So WFXT in Bristol County, MA is only useful for news and a couple of syndicated shows.
Yes, and you people are correct, WFXT is not on any significantly viewed list (even in counties in the Boston DMA, like Suffolk and Norfolk) because of the fact that they were owned by a religious company back in the hayday. WFXT could petition the FCC for significantly viewed status, but they have decided against doing so. If WFXT was instated as a significantly viewed channel here in Bristol County, MA, then Comcast would be able to lift the blackouts on network programming here on WFXT and we could watch the channel in its entirety.
Significantly viewed lists were (and still are) assembled based on over the air ratings. So if enough people watched a channel over the air, it was added to that county's SV list. Of course, this is an incredibly outdated procedure.
Now, another example is that WCVB is a SV station in Kent County, while WBZ is not. So if Cox could, they could carry WCVB on their cable systems in Kent County, including the ABC programming. Heck, WCVB would probably pile up more viewers in RI than WLNE ever did. However, if Cox decided to carry WBZ in Kent County, in which it isn't SV, Cox would need to black out the CBS programming on WBZ in honor of WPRI.
EDIT: I also wanted to point out that the significantly viewed list is a big reason why you see the Providence channels on Comcast on Cape Cod, despite the fact that the Cape and the Islands are all in the Boston DMA. Here is the SV list for Barnstable County, for example.
WBZ-TV, 4, Boston, MA
WCVB-TV, 5, Boston, MA (formerly WHDH)
WHDH-TV, 7, Boston, MA (formerly WNAC)
+WDPX, 58, Vineyard, MA (formerly WCVX, WZBU)
WLNE-TV, 6, Providence, RI (formerly WTEV)
WJAR, 10, Providence, RI
WPRI-TV, 12, Providence, RI
So Comcast can carry the big 3 out of Providence without needing to blackout programming. I don't believe that the Providence channels are in the HD lineup on the Cape, however. Here, in Bristol County, we get 2, 4, 5, 7, 38, and 44 on the Comcast HD lineup in addition with all of the major Providence channels. Neat, huh?