Gēng tiíold is engaged, and the eight-character post is exchanged between men and women. Name, date of birth, place of origin, three generations of ancestors, etc. It's all noted in the post.
Both "letting post" and "changing post" are similar to old wedding invitations. Generally, the post is handed in first, and Mr. Wang is asked to calculate whether it is suitable for marriage. If both parties agree, the posts will be exchanged.