The Coverage Level determines the number of items that your people will see for translation in each locale. The best target in general is modern: some companies that use CLDR data require coverage at a modern level before they will use the locale; others will use data that is less complete, such as moderate or basic. If it is not feasible to attain modern coverage in a given cycle, it is still best to set the coverage as high as feasible, such as moderate.
The number of items at each level varies by locale, but is roughly the following:
To see your current coverage levels, look for your organization in Locales.txt. It is a simple plaintext file, where each line is of the form:
<organization> ; <language_code> ; <coverage_level> ; <language name - optional>
A # starts a comment, and a * for the language_code means "all others".
An organization that focuses on a small number of locales will probably want to have just a line added for those locales, such as:
Breton ; br ; modern Breton ; * ; moderate
To request a change in the default coverage for your locale, file a new ticket.