In this search, structures can be searched that match exactly the query structure.
This can be adjusted to match exactly the SMILES ("Canonical Policy").
It can also be adjusted to match SMILES encoded stereo and isotope configuration ("Isotope and Stereo Policy").

In this search, substructures can be searched. meaning the given SMILE is part of the structure to be found.
Using the SMARTS language, wildcards for bonds and substituents can be set.

In this search, similar structures will be searched for. For the query structure a fingerprint will be generated.
For all sructures to query against fingerprints are available. The similarity is then calcualted as the Tanimoto coefficient between the fingerprints.
Since all structures will be queried and will end up in the result, a reasonable cutoff needs to be defined.