Lost earnings measure what the plaintiff was actually earning and would have continued to earn. Lost earning capacity measures the plaintiff's ability to earn, anchored in education, occupation, training, and labor market data — distinct from what was actually being earned at the time of the injury.
The choice between the two is jurisdictional and case-specific. Where the plaintiff was a minor, a homemaker, a student, or otherwise not yet realizing their earning capacity, the capacity measure is the appropriate frame.