It can also be implied by some other system parameter. As long as the same IV is never reused for the same key, it's safe. For example, if you're encrypting data blobs in some kind of CDN and you never mutate old blobs, your IV can be the blob ID.
That said, depending on the construction you use, nonrandom IVs can be a problem (they can leak that there's a relationship between different plaintexts if they line up with a difference in the first block), so you're better off at least making it a random function of the key and said blob/etc ID.
That said, depending on the construction you use, nonrandom IVs can be a problem (they can leak that there's a relationship between different plaintexts if they line up with a difference in the first block), so you're better off at least making it a random function of the key and said blob/etc ID.