As you know Bitcoind can run in pruned mode, thus not advertising blocks to other nodes. Even though it has some drawbacks, saving you storage space isn't a benefit you can ignore.

**My question is about the magic 550(MiB) default/minimum number. Why they've selected that limit? (& Why not less or more?)**

I know it implicitly indicates blocks will be kept for the last (about) 3.8 days (91 hours) (one MiB because of one block every 10 minutes). **But where that number come from?** I know there is also a dust limit equal to 546 Satoshis.[182*3 ~ 550 here! Even though it is unlikely to be related to this case]

Would you please explaining what ***the logic behind*** 550 is? Is there any kind of difficulty level relating to 51% attack or any other stuff like that?

**Edit**:
From 0.12.0, bitcoind supports relaying ***NEW*** blocks to other nodes, thus the first assumption up there, isn't an accurate sentence!