An acknowledgment records that someone confirmed they saw a procedure or a change, which is useful for an audit trail but says nothing about whether they understood it. Most people acknowledge changes quickly, especially when they get a lot of them.
A knowledge check asks the person to show they understand, usually through a few questions about the specific steps or changes that matter most. For critical procedures and changes to critical steps, a short knowledge check gives much more confidence than a signature, and it shows where people are actually confused.