The D ,T, P letters will signify the different package styles available, such as DIP, or surface mount in different variations.
Hard to say without knowing which manufacturer it is.
The B specifies that it is later (in time) version, versus no letter, or an A.
Schematic capture wise it probably does not make a difference which you use, the pinouts are the per this Fairchild data sheet as an example.http://www.futurlec.com/4000Series/CD4081.shtml
If you are modeling the part for physical layout, then it will make a difference.
If you are modeling the part for elecrical performance simulation I don't think it will make a difference, the part is pretty slow and does not show different parameters for one package type vs another.