There are no legal restrictions to what may be included in a Codicil to a Will. Some of the items which often involve Codicils include changes to executors, assigning an additional executor to be responsible for your social media and other online accounts, modifications to specific assets which you have bequeathed to other parties, or specifying who should take care of your pets upon your death.
In some instances, when marital situations change, a Codicil is used to add a new beneficiary to your will, including a child or a new spouse. Some people have used a Codicil to change beneficiary designations, as well. You can use a Codicil for any legal purpose to make changes to your Will at any time you feel is appropriate or necessary.