Encounter Slot is used to RNG wild pokemon. if you look at the Encounter Slot Table (there is a link in RNg Reporter, go up to, click 5th gen tools, then Black Encounter Table or White encounter Table depending of the version you use).
Pokemon are assigned encounter slots which hence the name you will encounter that pokemon that corresponds to the encounter slot. If you Look at Desert Resort on the Table, you will see that Encounter Slot 5 is where Level 20 Sigiglyph is, and only the slot is where Sigi can be found, thus if you hit another, for example slot 3 you will find a level 20 Darumakka or slot 11 you will encounter a lv 22 dwebble.
Pokemon are assigned encounter slots which hence the name you will encounter that pokemon that corresponds to the encounter slot. If you Look at Desert Resort on the Table, you will see that Encounter Slot 5 is where Level 20 Sigiglyph is, and only the slot is where Sigi can be found, thus if you hit another, for example slot 3 you will find a level 20 Darumakka or slot 11 you will encounter a lv 22 dwebble.