Answer
Double Class:- vessel have 2 class certificates, both Classification society will work independently to each other ie each will carry out their own survey as per requirements.
A double classed vessel is one which is classed by two Societies and where each Society works as if it is the only Society classing the ship, and does all surveys in accordance with its own requirements and schedule.
For Commercial attraction- Initially IRS was an associate member of IACS and hence shipowners needed to have IACS classed vessels to g ain commercial attraction.
Dual class:- vessel will have 2 class certificates, one classification society recognizes the survey carried out by other class.
A dual class vessel is one which is classed by two Societies between which there is a written agreement regarding sharing of work, reciprocal recognition of surveys carried out by each of the Societies on behalf of the other Society and full exchange of information on the class status and survey reports. It is taken when specified function are not carried out by the class. Eg. Ship with ABS class built in India& has IRS class also for ISPS certification.