Typically, you want to divide your input data into logical groups for analysis. In the examples with stocks, the pattern matching is divided so that it applies to just one stock at a time. To do this, use the PARTITION BY clause. PARTITION BY specifies that the rows of the input table are to be partitioned by one or more columns. Matches are found within partitions and do not cross partition boundaries.
If there is no PARTITION BY, then all rows of the row pattern input table constitute a single row pattern partition.
The ORDER BY clause is used to specify the order of rows within a row pattern partition. If the order of two rows in a row pattern partition is not determined by ORDER BY, then the result of the MATCH_RECOGNIZE clause is non-deterministic: it may not give consistent results each time the query is run.