When you install a septic system, you need to set aside land for the leach field or drain field. This is land that you won't be able to use for planting edible plants and installing certain structures on, and you need to know where the borders of the field really are so that you can make better use of all of the space in your yard. You can actually use the land on top of a properly installed and maintained septic system and leach field in certain ways, but if there are items you want that can't be placed on that field, you'll need to know where the borders are.
Err on the Side of Caution
Your household likely produces more effluent per day than you realize, which means a lot of liquid is flowing through those pipes from the tank into the field. That liquid is going to push out and away from the pipes as it filters through all the media you have packed around the pipes. Assume the drain field extends at least several feet away in all directions from these pipes. Anything that can't go on top of the leach field – plants with deep roots, heavy play equipment, aboveground pool, and so on – has to stay outside those borders.
Literally Mark the Corners
You should be able to get a diagram from the company that installed your septic tank and pipes showing where those items are in your yard. Ask the installers to help you mark the spots where the pipes are buried once the soil is back in place so you don't have to guess. Once you have those, you can measure out around the pipes and mark that perimeter. There are ways to permanently mark it so that you never have to guess where you can place that gazebo you want in your yard.
Landscape the Area for Better Drainage and a More Visible Warning
When you have a septic leach field, you don't want anything creating excessive pressure on the field (lest the installation work be ruined) or blocking sunlight from the field. When you have the septic tank and pipes installed, ask the installers what plants (grass, shallow-rooted shrubs and flowers, etc.) are suitable for the type of pipes they used and the depth they placed the pipes at. Use their recommendations to create a visible border around the field. You don't have to make this an impenetrable field; just place plants at various points that make it easy to tell where the field begins and ends. You may want to do this for the underground tank as well as the drain pipes.
Septic system installation is not just about getting the tank into the ground and hooked up to your home. It's about ensuring that you know where everything is and that what you do in the yard will help treat the effluent in the leach field. The company can show you everything you need to know as they install the system. Contact a septic system installation service near you to learn more.