As it happens, I was just working with a customer today on CLR assemblies, and it seems that the key/login is supposed to be used to "Sign" the assembly when it is built. The list that you have looks like it combines two different things: 1) creating an assembly that has been singed with a certificate or strong name(assymetric key), whose corresponding login has the EXTERNAL_ACCESS permission, as opposed to 2) Giving TRUSTWORTHY to the database and EXTERNAL_ACCESS permission to the database owner.

here is a microsoft link that explains: http://msdn.microsoft.com/en-us/library/ms345106.aspx