While creating Summery template,where do the letters like "D" comes from?

You will see three types of template values, T, D, and Rollups.

D simply means all detailed values would be retained in the template. T means amounts will be totaled for all values.

Lets say you have three segment structure as follows.

Company-Account-Department
01-1000-00
01-1100-00
02-1000-00
02-1100-00

Lets say you need to create summary accounts that will let you view combined balances for both companies together in various accounts.

You can create this template:
T-D-D

This template will result in these accounts:

T-1000-00 (will give you balance in account 1000 for both companies combined)
T-1100-00 (will give you balance in account 1100 for both companies combined)