The Wikipedia article they link to [0] has a pretty long list of brandnames. Many of them are the "-D" version of whatever the thing is. Claritin-D, Zyrtec-D and so on. I think the version without the "-D" doesn't have Pseudoephedrine?
In those cases, Claritin and Zyrtec are allergy medicines. The -D indicates that this allergy medicine comes along with a decongestant. The version without the -D will just be the allergy medicine.
Specifically, drugs with -D indicate that they come with pseudoephedrine. In the US, they (like the non-PE version of Sudafed) are only available behind the pharmacy counter.
[0] https://en.wikipedia.org/wiki/Pseudoephedrine#Brand_names