Matrix Management is quite common where diverse teams are required to make projects work.
As the Project Manager you will often have to pull together the required people into a team to be able to deliver the project. This could and is quite likely to be quite a diverse team.
If the project were to be ongoing for a number of years and you needed all of the team members 100% of the time, then you could probably justify employing a full time team, if this is not the case, then you could end up being in a matrix managed situation.
If you are pulling together a temporary team then it is unlikely that you will be given line management responsibility for those members, so even though you will probably be tasking them (managing) for the duration of the project, you will not have full line management responsibility for them, this is Matrix Management.