Subdomains are treated as separate domains by Google. Which means they need to rank and gain authority on their own. I highly recommend using sub-folders off the root. This will help with depth of content and link equity across the domian itself. Subdomains are appropriate when the content is not related to each other or when you plan on having a ton of pages/content to populate with for each of those sections. Depth of content and search friendly architecture is important to SEO.
Some operators need to use subdomains, because they are separately hosted applications that cannot be integrated into the rest of their offerings. Others use them because they believe their is a specific SEO benefit from subdomains … I do not.
@Simoneaton if you want to layout some specific examples of what you are looking to do, I can give some more targeted advice
thanks
scott polk