 The user defines a metric of interest, such as the validation error, and BO finds the optimal hyperparametersthat minimize it. However, the metric improvements on the validation set may not translate to the test set, especially  on small datasets.
<clarity>  The user defines a metric of interest, such as the validation error, and BO finds the optimal hyperparametersthat minimize it. However, the metric improvements on the validation set may not translate to the test set, especially  on small datasets.
 The user defines a metric may not translate to the test set, especially on small datasets.