Invisalign is a clear aligner company that moves teeth using custom-made clear plastic trays. It can be used on both upper (maxillary) and lower (mandibular) teeth and in a wide range of clinical presentations.
However there are some clinical presentations where clear aligners are not as efficient and your dentist/orthodontist may recommend fixed appliances on the opposing arch. These may be:
1. large spaces that your dentist is planning to close
2. deep overbite where there is a greater vertical overlap between the upper and lower front teeth
3. teeth requiring a lot of vertical eruption (i.e need to be pulled downwards a lot)
4. very small teeth
However, there may be other reasons that are more specific to your situation that I have not mentioned. If you have any doubts, do highlight it to your dentist/orthodontist and I am sure he/she will be most happy to assist you.