You’ve applied for a shift which has been published by the client. Everything that was mentioned in the description has been agreed upon in the shift-agreement. If you’ve been hired as a ‘barista’ the client can’t put you in the kitchen to go and clean the dishes if this was not previously mentioned in the shift. 

The client can always consult with you, whether you would like to perform other jobs or stay 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 your rating. You can give them feedback that they should add this to the shift description beforehand so that a FreeFlexer knows what to expect.