Hi Gracie
Any clip, whether it is in the clip bin or any of the timeline tracks can be selected and previewed in the Clip Preview window. If the clip in question is only in the Clip bin and you click it, it will appear in the Clip Preview window, but Not in the Sequence Preview window which only previews the timeline. In a sense the clips you have added to the timeline are the only ones you have "opened" You can delete any of the clips in the clip bin as Borate says (opened" or not but if you have "opened" them (i.e. put them on the timeline), VP will ask you if you want to do this action as it will also remove the clip from your sequence. If, on the other hand you select the clip on the timeline and delete it, it will be removed but remain in the clip bin.
As you edit your video....add clips to the timeline and then split and add effects etc. The steps you perform are saved in the background by VP. If, at some point you Save the project, VP creates a text file containing the names and locations of all the clips you have in VP as well as details of the effects, edits etc you have made. This is given a name by the user (e,g, "My Video") and is saved in a designated folder as My Video1.vpj VP also saves copies of all your loaded clips as well as thumbnails and stills in a Cache folder.
When you open My Video1.vpj VP reads the text data and then recreates the project using the files saved in the cache.
Now.....if you continue to edit and then use Save project once more, My Video1.vpj is overwritten by a new My Video1.vpj You won't get the opportunity to alter anything the old file is simply overwritten and your original edit and its .vpj file is lost. This is OK in some respects but it is sometimes useful to be able to go back to the previous edit.
If you think you may want to do this then you should use the option to Save Project As. This allows you to enter a new (and different name) ...e,g, My Video2. There is no overwriting and you will have a second .vpj file to return to.
If you did this then you should have a .vpj file called project-2.vpj (somewhere)
Nat