A customer rarely buys after one ad. They might see a post, search for your brand, read an article and then click a retargeting ad. Attribution is the rule that decides which touchpoint gets the credit.
Last-click
Last-click gives all the credit to the final interaction before a conversion. It is simple and easy to explain, but it undervalues the channels that start the journey, such as social or content.
First-click and linear
First-click credits the first touch. Linear splits the credit evenly across every touch. Both are blunt, but they can show what starts demand and what supports it.
Data-driven
Data-driven models use your own conversion data to estimate how much each touchpoint contributed. They need enough conversions to learn from, so small accounts often cannot use them well.
What to do in practice
Pick one model for day-to-day optimisation, check another for the broader view, and never treat any model as the full truth. Attribution estimates the story; it does not prove cause.