Many things can happen based on available data model such as -
1. Exists with single parameter can load QVD in optimized load,with two parameters it loads QVD in bit slow/standard mode.
2. Exists(Single Field) looks the values in symbol table so totally independent of specific tables loaded, if you want comparison with a specific table, you may want to rename the loaded field to tempfield(so it links to only one table) in that table and then use ( Where exists(tempfield,newlyloadedfield) while loading new table, the search here as well happens in symbol table but into the tempfield symbol table which we know available in single table only.
3. Where not exists(single field), it cannot load duplicate values, because 2nd and later comparisons will fail as value already exists in symbol table.
