Identify an author's purpose (to inform, persuade, or entertain) and explain how angle -- which details are chosen and how they are framed -- serves that purpose.
Every text is made of choices. A writer decides what to include, what to leave out, and how to frame what stays in -- and those choices always serve some purpose, whether to inform, to persuade, or to entertain. Angle is the specific slant those choices create. Angle is not automatically dishonest; even a fully accurate account has to choose which true details to feature.
You do not need words on a page to see this. The Bayeux Tapestry, made soon after 1066, is a nearly 230-foot embroidered strip telling the story of the Norman conquest of England -- and it was almost certainly commissioned by someone on the winning Norman side. Its angle shows: it features a scene of the English king, Harold, swearing an oath to support William of Normandy's claim to the throne, then shows Harold breaking that oath by taking the crown himself. Featuring that broken oath so prominently serves a clear purpose -- justifying William's invasion as rightful payback rather than naked conquest.
Mastery looks like: They can name a text's likely purpose and point to a specific chosen detail (inclusion or omission) that serves that purpose.
Common stumbles: Treating "angle" as automatically the same as "lying," rather than recognizing that any account, even an accurate one, must choose an emphasis.