> The username is a unique identifier of your account on this and all the other instances.
this is true if we consider "john@domain.com" as "an account". If by "an account" you only mean "john" then the text needs to be changed to something like:
> The username is a unique identifier of your account on this instances. Together with the domain name - it becomes unique across all instances.
In any case, English native speakers I'm open to suggestions on how to improve this :)
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<div class="name-information" i18n>
- The username is a unique identifier of your account on this instance. It's like an address mail, so other people can find you.
+ The username is a unique identifier of your account on this and all the other instances. It's as unique as an email address, which makes it easy for other people to find it.
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<div *ngIf="formErrors.username" class="form-error">