Field help (AOBSELCAR) 

Any field in a screen where the type is linked to a object has available a selection field accessible by right click or by.

This value is used to define the number of characters used to specify where the selection should start, as a function of what has been entered in the field before the selection. If this value is N, the selections starts from the first key where the first N characters are greater than or equal to the string entered. Indeed :