Each class does things in a slightly different way. The upper class have their balls and the lower class their ‘knees ups’. Knowing which fork to use or how not to look the local gangster in the eye are both forms of etiquette. However, if you are used to one and not the other you will run into problems.
When you are interacting with someone from a different social class, you apply a -4 penalty to the roll. This represents the differences the classes have about how things should be done. Should you fail a roll, the consequences will depend on the company you are keeping. A lower class character failing an etiquette roll in the company of their betters will be thrown out for rudeness, while an upper class character failing a roll to garner information from their inferiors might only find stony silence.