You’ve applied for a shift which was published by the client. Everything that was mentioned in the description has been agreed upon in the assignment agreement. If you’ve been hired as a ‘barista’, the client can’t put you in the kitchen to clean the dishes, if this was not previously mentioned in the shift.
The client can always consult with you, and ask if you would like to perform other jobs or work overtime. It’s up to you whether you agree with this or tell them that this is not what you agreed upon in the contract. Please keep in mind that your new jobs will be taken into account once the client will be giving you a rating. You can give them feedback that they should add this to the shift description beforehand so that FreeFlexers know what to expect.