For the classic drinks like Manhattan, Martini, Gimlet, etc... there is a generally accepted base receipt. When someone walks into a bar and orders one of these the barkeep should know what is expected.
Example: Manhattan - bourbon, sweet vermouth, bitters, which is stirred with ice and strained into a cocktail glass.
This is the default receipt. No question. If a patron asks for this cocktail they most likely know what they are asking for. If not, shame on them.
If the patron wants something different then, they specify. Example: Sweet Manhattan on the rocks.
This would let the bartender know that you want bourbon, sweet vermouth, bitters, with a splash of cherry juice in an old fashioned glass over ice.
Without a known base receipt that both bartender and patron know there is miscommunication. This will result in the patron not getting the drink they want and the bartender not getting the tip they want. It's a vicious cycle.
Now, over time base receipts change. Back in the day Manhattan's were made with Rye instead of Bourbon. This was because the Manhattan was actually invented during prohibition days and that was what was available. Since then Rye has become less popular and Bourbon took its place in this classic drink.
The Martini is currently going through a transition. The base receipt for a Martini calls for Gin. However, more and more people are expecting Vodka when they order a Martini. If you want Vodka, ask for a Vodka Martini. We may come to the day when Vodka becomes the default ingredient for a Martini, but we are not there yet and I know many people will fight this for years to come.
If you are bartender, learn the classics.
If you are the patron, don't complain if you order a Martini and you don't get your vodka.