This is what I understand:
- if you are already on ESXi 5.5 Update 1, only use "ESXi550-201404001"
- if you are on a previous build, you can use either "ESXi550-201404020" or "ESXi550-201404001".
One of the reasons not to patch a current non-Update 1 host to Update 1 is the NFS issue mentioned in the KB.
André