An opening remark would be one to broach a topic to be elaborated on, whilst a welcoming one would be to initiate an event by welcoming everyone there.
Here’s a conceptual PTM as an example.
Welcoming remark - I would like thank everyone for being here tonight for this very special event, especially to the members of Evan’s family who I know have had to travel a long way to be here.
Opening remark - As most of you know, there have been a number of serious events in the school this last few weeks and we would like to take the opportunity to take you through some of the measures that have been taken to ensure the safety of your children and our school.