Agents are molecular structures that do not take part in the chemical reaction, but are added to the reaction equation for informative purpose only.
All of the above sections are optional. For example:
The "unique" name can be sometimes misleading when dealing with compounds with stereo centres.
Daylight's SMILES specification (3.1.SMILES Specification Rules) defines generic, unique, isomeric and absolute SMILES as:
The name canonical SMILES is used for absolute or unique SMILES depending whether the string contains isomeric information or not (both strings are "canonicalized" where the atom/bond order is unambigous).
Marvin generates always canonical SMILES with isomerism info if it is possible to find out from the input file. The molecule graph is always canonicalized using the algorithm in article  but it is not guaranteed to give absolute SMILES for all isomeric structures. The unique SMILES generation (option u ) currently uses an approximation to make the SMILES string as absolute (unique for isomeric structures) as possible. In this case the form of any aromatic compound is aromatized before SMILES export. For correct exact (perfect) structure searching MolSearch and JChemSearch classes of JChem Base or the jc_equals SQL operator of the JChem Cartridge are suggested.
The initial ranks of atoms for the canonicalization are calculated using the following atom invariants:
See ref.  for details.
With option u it is possible to include chirality into graph invariants. This option must be used with care since for molecules with numerous chirality centres the canonicalization can be very CPU demanding .
SMILES canonicalization algorithm is not generic, it depends on the software package, so it is most useful to compare SMILES strings within a software package.
|||SMILES 2. Algorithm for Generation of Unique SMILES Notation; D. Weininger, A. Weininger, J. L. Weininger; J. Chem. Inf. Comput. Sci. 1989, 29, 97-101|
|||A New Effective Algorithm for the Unambiguous Identification of the Stereochemical Characteristics of Compounds During Their Registration in Databases ; T. Cieplak and J.L. Wisniewski; Molecules 2001, 6, 915-926|
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