Hi,

Based on the account. move xml code if the state comes to draft the Invoice sequence comes Draft (Draft) so when we create a record in the invoice (account.move) and its comes draft state the Customer Invoice filed occur as the text Draft and it comes poster stage that generates the sequence number.

We can add any name in the draft state - the Draft text is only like a placeholder. 

We can add any names here and we can post it with the same name. But in the odoo default feature, it will take a sequence number format

When we change the sequence it shows like this 

In general case, the number field comes as read-only, and we set the value like a sequence number and it applies in the Posted stage 

Hope it helps