diff --git a/spa/include/spa/buffer/buffer.h b/spa/include/spa/buffer/buffer.h
index db52bc56d..09f012579 100644
--- a/spa/include/spa/buffer/buffer.h
+++ b/spa/include/spa/buffer/buffer.h
@@ -63,7 +63,9 @@ struct spa_chunk {
 
 /** Data for a buffer this stays constant for a buffer */
 struct spa_data {
-	uint32_t type;			/**< memory type, one of enum spa_data_type */
+	uint32_t type;			/**< memory type, one of enum spa_data_type, when
+					  *  allocating memory, the type contains a bitmask
+					  *  of allowed types */
 #define SPA_DATA_FLAG_NONE	 0
 #define SPA_DATA_FLAG_READABLE	(1u<<0)	/**< data is readable */
 #define SPA_DATA_FLAG_WRITABLE	(1u<<1)	/**< data is writable */
diff --git a/src/pipewire/buffers.c b/src/pipewire/buffers.c
index f56f08dbc..4f685f7b9 100644
--- a/src/pipewire/buffers.c
+++ b/src/pipewire/buffers.c
@@ -103,6 +103,7 @@ static int alloc_buffers(struct pw_mempool *pool,
 			d->maxsize = data_sizes[i];
 			SPA_FLAG_SET(d->flags, SPA_DATA_FLAG_READWRITE);
 		} else {
+			/* type is a bitmask of allowed types */
 			d->type = SPA_ID_INVALID;
 			d->maxsize = 0;
 		}