This warning appears as shown below.
The Beta Ratio is the orifice diameter ratio (the ratio of orifice plate bore divided by the upstream internal pipe diameter) where β = d/D where d is the orifice plate bore in mm and D is the inside diameter of the pipe in mm. Note, a beta ratio of say 0.4 denotes that the orifice bore is approx 40% of the pipe inside diameter. The recommended ratio limit according to ISO 5167 is 0.15 to 0.68.
If we consider an orifice plate in a flow stream, an increase in fluid velocity through the orifice bore (reduced area) develops a differential pressure across the orifice plate. The differential pressure is related to the beta ratio of the plate. The smaller the beta ratio the higher the differential pressure generated. In practice, the selection of beta ratio is a compromise between the differential pressure desired and the flow rate required in the system.
To ensure conformance with recommended practice, the beta ratio should not exceed the recommended limits as noted in the warnings/messages. Exceeding the recommended limits in real systems can lead to lower levels of accuracy at measurement points.