Usually meaning is that if you were to pressurize a sealed container, it would reach that pressure (as long as the seal does not break). Similarly a maximum flow rate is how much would flow if there were no resistance (pretty much never).
Where there is flow, there is resistance. The faster you need something to flow (against the walls of something), the greater this resistance. It is also affected by other factors like the viscosity of the fluid and the diameter of the hose/pipe you are flowing it through. In other words if it doesn't get the rate you need, make changes to things downstream from the pump. The literal bottleneck may be that of your target container. It should be reasonably easy to test the resistance for your setup using gravity instead of a pump, and a stopwatch. How high (compared to where you'd connect the pump) does your source fed through your pipe need to be to get your 1L filled in 10s?
Whether you need the higher pressure pump will depend on the pressure vs flow curve of the pump - usually it's more than safe enough to assume that you can draw a line from 0 flow and max pressure to 0 head and max flow. This is because of the shape usually being convex to the 0,0 point. Take your head from the height experiment (0.0361 psi per inch water), and if your flow on that line is greater than 6L/min you can consider it OK.