[garc_mall]

  If I brew a beer with an og of 1.050 and an fg of 1.012, and use a yeast that is estimated at 75% att. That's a fully attenuated beer.
If I brew the same beer with a yeast that is rated at 60% attenuation and I achieve that 60% winding up with a fg of 1.020. Is the beer underattenuated because it could go further, or is it fully attenuated because it has reached the limit of the yeast? And wouldn't it be sweeter because there are more unfermented sugars?
  By the same token if I brew the same beer but use a higher mash temp. so the yeast rated for 75% only attenuates to 60% is that fully attenuated because it's reached it's limit due to less unfermentables.
 To much time off from work is leaving me to much time to think about all this.

The beer is underattenuated if there are fermentable sugars left after yeast activity is "finished," regardless of the numbers.

The yeast "rating" is based off a general (lab-specific) wort, and is really only comparable on a general level (low attenuation, high attenuation), and I don't trust the numbers given.

If I felt I had an underattenuated wort, I would probably do a forced ferment test, and see if it could go any lower. When I taste beers that are underattenuated (I know a small micro that cold crashes about 3/4 through fermentation to "preserve the sweetness") I generally get that from extra sweetness that tastes like it isn't supposed to be there.

[morticaixavier]

  If I brew a beer with an og of 1.050 and an fg of 1.012, and use a yeast that is estimated at 75% att. That's a fully attenuated beer.
If I brew the same beer with a yeast that is rated at 60% attenuation and I achieve that 60% winding up with a fg of 1.020. Is the beer underattenuated because it could go further, or is it fully attenuated because it has reached the limit of the yeast? And wouldn't it be sweeter because there are more unfermented sugars?
  By the same token if I brew the same beer but use a higher mash temp. so the yeast rated for 75% only attenuates to 60% is that fully attenuated because it's reached it's limit due to less unfermentables.
 To much time off from work is leaving me to much time to think about all this.

The beer is underattenuated if there are fermentable sugars left after yeast activity is "finished," regardless of the numbers.

The yeast "rating" is based off a general (lab-specific) wort, and is really only comparable on a general level (low attenuation, high attenuation), and I don't trust the numbers given.

If I felt I had an underattenuated wort, I would probably do a forced ferment test, and see if it could go any lower. When I taste beers that are underattenuated (I know a small micro that cold crashes about 3/4 through fermentation to "preserve the sweetness") I generally get that from extra sweetness that tastes like it isn't supposed to be there.

Right,

the yeast attenuation numbers are guidelines and do not reflect the real world all that well. I've had low attenuators finish >75% and high attenuators that finish <70%. There are some yeasts strains that seem able to ferment sugars that others are simply not capable of. Actually there are DEFINITELY strains like that. Ale yeast in general is largely defined by it's ability to ferment rafinose while Lager yeast can't. (I think i got that right  but smarter folks will chime in to correct me if I didn't, ITIGTRBSFWCITCMIID?) similarly brett will ferment WOOD if given enough chance and nothing else to eat. But with the standard ale yeast species it's not as clear cut as AA% would lead you to believe

[hopfenundmalz]

The CAMRA books I have have the mash temp for milds at 153F, but those generally have other things like a god amount of crystal malt or wheat to aid the body. If you look at the NHC award winners that were milds those go to 156F.

I also will be contrarian in that if I want a malty beer I do that with the malts selected, and malty does not equal sweet. You can have a malty beer that is fully attenuated, Oktoberfests come to mind.

For some British beers, more crystal is not out of place. One should also think about mild ale malt, amber malt, and broom brown malt for some of those beers. Bitterness levels and water chemistry can also be adjusted to give a dryness to a beer.

Uncle Jeff's $0.02 on this.

[ynotbrusum]

The CAMRA books I have have the mash temp for milds at 153F, but those generally have other things like a god amount of crystal malt or wheat to aid the body. If you look at the NHC award winners that were milds those go to 156F.

I also will be contrarian in that if I want a malty beer I do that with the malts selected, and malty does not equal sweet. You can have a malty beer that is fully attenuated, Oktoberfests come to mind.

For some British beers, more crystal is not out of place. One should also think about mild ale malt, amber malt, and broom malt for some of those beers. Bitterness levels and water chemistry can also be adjusted to give a dryness to a beer.

Uncle Jeff's $0.02 on this.

Agreed Jeff.  Think Bock - it is fully attenuated, but malty.

[denny]

Agreed Jeff.  Think Bock - it is fully attenuated, but malty.

Or Ayinger dunkel.