\subseteqThe symbol \subseteq is used to denote that one set is a subset of another set, meaning that all elements of the first set are also elements of the second set. It is a relation between two sets.
Set A is a subset of set B, meaning every element of A is also in B.
A \subseteq BThe set containing elements 1 and 2 is a subset of the set containing elements 1, 2, 3, and 4.
\{1, 2\} \subseteq \{1, 2, 3, 4\}The empty set is a subset of any set A.
\emptyset \subseteq A