Placeholder is a little-noticeable inscription in input fields when their value is listed as empty.

To add a placeholder, you need to add a new one with placeholder_ prefix for the selected language in the messages. For example, there is a section describing messages in English for resellers in DNSmanager:

...
  <messages name="reseller.edit">
    <msg name="domainlimit">Domains limit</msg>
    ...
    <msg name="placeholder_domainlimit">99</msg>
    ...
  </messages>
...

As a result, when we start to add a reseller, we will see a corresponding message as a placeholder for the field to limit the number of domains.