The UVPackmaster plugin uses island similarity detection in the following situations:
Accuracy of similarity detection depends on two factors:
Similarity Threshold parameter (in the Advanced Options rollup): a greater value of this parameter means islands will be more likely recognized as a similar in shape. A lower value means more accurate distinction. 0.5 is a good threshold value to start with.
Precision parameter (in the Basic Options rollup): more precision means better accuracy in looking for similar islands. Precision set to 500 should be sufficient in most cases. This value should be increased in the first place, when dealing with really small islands.