Planning time fence is the period within which, during planning, there is no system intervention in terms of creating or deleting already existing Reciept elements
So if you have a Planning time fence of 10 days, then nothing that has been in planned in previous planning runs and now inside the 10day time fence will be changed by the new planning runs. (You can, of course, change it manually)
PPDS horizon is the period within which the planning runs will create PPDS orders. So if you have a 30day PPDS horizon, then on running any PPDS planning you will get PPDS planned orders only inside this 30 day time period
So if you have a 10dday Planning time fence and 30 day PPDS horizon and run planning you will create new orders for day 10 to 30
SNP Horizon is the period OUTSIDE which the SNP application creates orders.
if you dont maintain PPDS horizon, SNP horizon will be the PPDS horizon as well. you can also have shorter SNP horizon than PPDS horizon (for eg 25 days) and there will be a (5 day in this eg) period of overlap during which both SNP and PPDS can happen
SAP note 481906 - SNP - PPDS integration documentation is a good note that has some sections that will explain more about this in detail