The type must work with < operator and the result should have standard semantics.
The type T
satisfies LessThanComparable if.
Given.
a
, b
, and c
, expressions of type T
or const T
The following expressions must be valid and have their specified effects.
Expression  Return type  Requirements 

a < b  implicitly convertible to bool  Establishes strict weak ordering relation with the following properties

To satisfy this requirement, types that do not have builtin comparison operators have to provide a userdefined operator<.
For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between equality, which is the value of the expression a == b
and equivalence, which is the value of the expression !(a < b) && !(b < a)
.
a BinaryPredicate that establishes an ordering relation (named requirement) 
© cppreference.com
Licensed under the Creative Commons AttributionShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/named_req/LessThanComparable