Q:

What is the difference between using "of=/dev/sdX" and "of=/dev/sdXY" in dd

First of all, this question is NOT the duplicate of this one, which compares lower-case "sdx" with mixed-case "sdX". Back to the question.
If I have a formatted memory stick which is composed of only a single partition and I use the following
dd if=input_file of=/dev/sdXY bs=ZZ ...

How will it be different from using 
dd if=input_file of=/dev/sdX bs=ZZ ...

The question came to my mind because when i had to make a bootable Linux USB, I had to mention the drive number and NOT the partition number i.e.
dd if=Linux.iso of=/dev/sdX bs=ZZ ...

Would be glad if someone can explain the difference while considering one single partition drive/stick etc.

A:

Most ISOs nowadays are "hybrid ISO" images containing CD/DVD boot code (El Torito standard, 
an extension to the ISO 9660 CD-ROM standard) and 
an MBR (including boot code). This makes them bootable from CD/DVDs or USB storage devices.
By copying the ISO to the device (and not a partition), you create a USB bootable version of the ISO (like the read-only version on optical media) including 
MBR and partition(s).
If you copy an image to a partition, you generally copy a "partition image" (without MBR / GPT).

