The AllowPreserveRatio specifier, which applies only to FVector properties, adds a ratio lock button (indicated by a padlock) to the details panel display. Most users will likely have seen this specifier in action for an Actor's Scale property.
The lock can be toggled on and off. When enabled, changes to one component of a vector will be proportionally applied to the other components of the vector. Consider the following vector.
UPROPERTY(Meta=(AllowPreserveRatio="true"))
FVector Scale; // X=1.0, Y=2.0, Z=3.0If the lock is enabled and Scale.X is manually set to 2.0 (an increase of 100%), the remaining values of Scale.Y and Scale.Z will be automatically set to 4.0 and 6.0, respectively (after applying 100% increase). This can be very useful when resizing meshes or in other contexts where you want to preserve the ratio of vector components.