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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.

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    There are important 'files' on the harddrive outside of the partition, such as the master boot record (or MBR for short). Commented Aug 26, 2019 at 12:20
  • Partition table and the partitions it contains is a data Structure, stored on a storage device. You just needs to know what the data structure looks like Commented Aug 26, 2019 at 13:51

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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).

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