MySQL 9.0.0
Source Code Documentation

The return type of row_compare_row_to_range() which summarizes information about the relation between the row being processed, and the range of the scan. More...
Public Attributes  
bool  row_can_be_in_range 
true: we don't know, false: row is not in range More...  
bool  gap_can_intersect_range 
true: we don't know, false: gap has nothing in common with range More...  
bool  row_must_be_at_end 
true: row exactly matches end of range, false: we don't know More...  
The return type of row_compare_row_to_range() which summarizes information about the relation between the row being processed, and the range of the scan.
bool row_to_range_relation_t::gap_can_intersect_range 
true: we don't know, false: gap has nothing in common with range
bool row_to_range_relation_t::row_can_be_in_range 
true: we don't know, false: row is not in range
bool row_to_range_relation_t::row_must_be_at_end 
true: row exactly matches end of range, false: we don't know