The site name is the unique name in the list to find the site. Since each password must have a unique name, it's not possible to have 2 twitter.com passwords without giving them a unique name, so if you wanted separate passwords, you might prefix the name with something to make them unique. Our suggestion is to use the username for this purpose since the differentiating factor tends to be "separate accounts" which automatically implies separate account usernames.
A site can generate different types of tokens, such as password, login name, security/recovery answers. Each of these tokens can be generated using our algorithm or an encrypted custom token saved in your site configuration. As a result, for each site entry in your list you can generate or save a login name. This is separate from the name you'd use to differentiate the entry in your list, but you might choose to use the same word in both places.
So, [email blocked], as the name of your site, is only used to allow yourself to create multiple mysite.com entries in your list; nothing else.
Saving a login name in that site, you could save myusername in there as well, or you could set it to something different, or you could let the app generate a username for you. This login name can easily be copied out of the site for use in web forms, unlike the prefix in your site name.
For most modern sites, login credentials are a username / password pair,
the username and login name usually being the same,
and to save the user from creating multiple tokens - the username and login
name are set to be the user's email address.
This is the most common case I came across so far.
For the case where a username and loginname can be different, then it makes
sense to use the generated loginname as an added layer of obfuscation.