There are so many things a doula will help with during pregnancy, labor, birth, and postpartum. A doula will provide excellent resources for you (like local childbirth classes, mental health services, where to go if you have difficulty breastfeeding, and the best websites and books for information), give endless emotional support and strategies for comfort during labor, get you started off right with breastfeeding, and make sure you're doing well postpartum. There are, however, certain things that a doula will never say to you, which is key to her role, scope of practice, and professionalism. It helps for you to know these things prior to hiring a doula for your birth.
If/when to get an epidural - If your goal is to birth without pain medication, a doula will support you endlessly with encouragement, coping techniques, position changes and more. If you come to a point in labor, however, where you have absolutely reached your limit for whatever reasons and have changed your mind, the choice to accept pain medication is yours. A doula will not tell you when she thinks it's time or conversely, sway you away from pain medication when you make that decision. A doula supports without judgement -- it's not her job to decide if/when you get pain medication, nor will she be upset about your change in course (changes in labor and birth happen all the time!).
Your doctor/midwife is terrible - Or that she thinks you should switch providers. Or that she can't stand working with your provider. This is not to say that a doula will leave you in the dark about a provider with a high rate of c-sections or who is known to induce nearly everyone. A doula will provide you with intel about your provider in a diplomatic way, and perhaps more importantly, will arm you with questions and discussion points -- if desired -- to find out if your provider is the ideal match for supporting your desires and goals in birth.
Anything that involves a diagnosis - A doula cannot and will not tell you anything that even remotely sounds like a diagnosis. You can discuss all the symptoms you want with your doula -- she will advise you to call or visit your care provider, seek medical care right away, or provide you with resources to learn more about your symptoms.
Who you should/should not allow at your birth - During prenatal meetings, your doula will ask about who will be present with you at birth -- for informational purposes only. A doula will not tell you it's a bad idea to invite your mother-in-law, with whom you've had a contentious relationship. She will, likely, suggest to strongly consider who will be present at your birth. If asked for, she may also provide advice on how to politely request someone not attend your birth.
That you're really in labor -- or not - The definition of "true labor" is contractions that grow longer, stronger, and closer together. But labor is a funny thing and doesn't always follow a direct path or pattern. Sometimes, early labor and active labor can be full of stops and starts. So while a doula is well versed in labor signs, and can help you assess where you are, the beginning of labor is not always a hard-and-fast science. Of course, it also helps to know that all contractions and signs of labor, while sometimes referred to as "false labor," are all an important part of the process.