bar and cover tolerance

huangyhg

bar and cover tolerance
aci 318 d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement, in.
aci 318 7.5.2.1 鈥?tolerance for depth d, and minimum concrete cover in flexural members, walls and compression
members shall be as follows:
(see table)
except that tolerance for the clear distance to formed
soffits shall be minus 1/4 in. and tolerance for cover
shall not exceed minus 1/3 the minimum concrete
cover required in the design drawings or specifications.
can anyone give a less complicated explanation of d and what the tolerance means.  i think it means the depth from the top of the beam to the center of the bottom bars.
also, how can the tolerance for d have a plus and minus but tolerance for cover only have minus?
i would not concern myself with the placement tolerances.  all the tolerances are doing is limiting the erector from making a poor concrete section that will not behave as designed.  just use a "d" from the extreme compression fiber to the centroid of your steel.  
a soffit is the underside of a structural component.  like the underside of a canopy.
explain the material as an inspector would use it, not a designer.
the location of the stel could be up or down.  one increases your cover, the other makes it less.  the minimum cover insures just that, a minimum.  if you have more, that is ok, from a cover standpoint.