Ikhtiṣār and ījāz express substantial meaning with economical wording. The surrounding context, known conventions, or earlier discussion may make it unnecessary to restate every detail.
A concise statement does not authorize the interpreter to add any detail one happens to prefer. Only what is supported by language and evidence should be supplied.
Ellipsis and concision are related but not identical: ellipsis may require identifying a specific unstated element; concision is a wider rhetorical quality in which few words effectively express a rich intended meaning. The two can overlap.