Please note that this currently only applies to reseller plans on our older hosting system. We do not currently offer reseller plans on our new shared hosting packages.
Whether you are using DirectAdmin or Helm your clients will be able to login to manage their website if you have a reseller package on our older system.
If you are using a Plesk VPS to resell hosting you can set up "Clients" to log in to a number of domains with their own control panel, or else you can enable a user on a per domain basis to have access to a control panel for just that domain. You may want to refer to the Plesk documentation for more information on this.