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.
a == b
and !(a < b) && !(b < a)
. a BinaryPredicate that establishes an ordering relation (named requirement) 

(C++20)  specifies that a relation imposes a strict weak ordering (concept) 
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