Dedicated launch services will carry one single Mini- or Micro-Satellite. You as the single customer determine launch specifications and therefore have full control of orbital parameters and schedule. A dedicated cluster with several identical payloads is also possible.
Rideshare launches carry payloads of different mass, volume and customers on one launch vehicle. This configuration does not have a major customer who defines the orbital parameters or the time slot. It is an analogy to a shared taxi trip to space. The launch service provider initially defines the launch date and the orbital parameters. Changes might occur from considering customer needs, if a majority is present with similar targets.