You can create blend shapes with target objects whose topology differs from the base object using the Check Topology option in the Blend Shape Options. This option checks that the base and target objects have the same number of vertices.
If you're using identical multiple target objects to create a blend shape (Create blend shapes using multiple target objects), you should turn on the Check Topology option when you create the blend shape deformer (Create blend shape deformers).
If you're using target objects that have a different number of vertices than the base object, you should turn off the Check Topology option. However, If the vertices are different and this option is off, you might see some unusual blending, such as an eye blending into the nose.
If the target and base objects have the same number of vertices but their order is different, Maya blends the shapes whether Check Topology is on or off. However, the position of the base object's vertices will be transformed to the position of the target objects' vertices. This change might cause the object to blend in a way you might not expect. To ensure a smooth transition between base and target, make sure the order of vertices is the same in both base and target objects.