Patterns contain rhythm and melody information. Each pattern has 8 tracks of information.
Parts contain information about machines and samples to be played. (Parts are like “kits” on the other Elektron instruments.) Each Part contains the information for the machines on each track.
Save the Part with the pattern that you first create. When you want to use new samples, switch to another pattern and save another Part.
Make sure that you read the manual for the OT until you understand it. It will help if you read Merlin’s “Some thoughts on Elektron’s Octatrack” document.