SharePoint content crawling is limited by the number of sources and repositories you are crawling.
If you have multiple kinds of content repositories that you want to crawl, or you want to crawl some content repositories on different schedules, you have to create additional content sources. Office SharePoint Server 2007 can support up to 500 content sources per Shared Service Provider (SSP), and each content source can contain up to 500 start addresses. (You cannot crawl the same address using multiple content sources. For example, if you use a particular content source to crawl a site collection and all its subsites, you cannot use a different content source to crawl one of those subsites on a different schedule)
You can only crawl one kind of content repository per content source. That is, you can create a content source that contains URLs for SharePoint sites and another that contains URLs for file shares. But you cannot create a single content source that contains URLs to both SharePoint sites and file shares.