It's not cut and dried in that way ("certain output"), in many models, any one piece of training data can influence any number of weights and potential outputs,

It's not cut and dried in that way ("certain output"), in many models, any one piece of training data can influence any number of weights and potential outputs, and it's difficult or impossible to say which output was influenced to what extent by which input. A drop of blood falls in a lake.. which bit of lake now contains blood?