You are right, it is a MS SQL Server thing. I did a GOOGLE search using SQL and PLAN_ROW and it returned a Microsoft Library page on SET SHOWPLAN_ALL which mentioned 'PLAN_ROW type'. On that page there is a table which had the following description for Type:
Node type. For the parent node of each query, this is the Transact-SQL statement type (for example, SELECT, INSERT, EXECUTE, and so on). For subnodes representing execution plans, the type is PLAN_ROW.
That last line is the only description I could find for PLAN_ROW. Hope that helps.
-SQLBill