As you will know, term sets allow users to create collections of terms, which it can be reuse to associate content. In this case, usually, the SPO admin or the site owner prepopulate the Term set for all the users. But what happen when we have created a term set and we want to allow the users to add their own values into an specific term set?
In one hand, we can talk about open and closed term sets, by default, term sets are closed, so the term manager, must create the term and users will need to use them. There is no possibility to add new terms
On the other hand, we have open term sets, in this case we allow users to add their own values to the term set from input forms, it’s like to say: Ok, I’m allowing to create new terms on the fly
This option is quite great, but you have to be careful, because this option probably will not fit with your SharePoint governance strategy…