The startup flags or startup commands are the instructions you give the server on how to boot your game. Game developers sometimes allow you to change settings by using the startup command, or for games like Minecraft, you can sometimes improve performance by selecting the correct settings.
For most games that are not Minecraft. You can change any startup settings in the quick settings options on the overview page.
For Minecraft (Java), you'll need to select flags with the startup flags drop-down.
No matter what you're running we recommend you select the Aikar flags. These are the most optimised settings for Minecraft and can help with memory crash issues, and allow your server to run smoother.
You can select these flags in the drop-down. Make sure you select the correct option for your RAM value. Otherwise, you will get memory crash issues.