在Visio数据库关系图中，I1、I2、U1是什么意思?

I am reverse engineering an database to create an database ER diagram in Visio. When I display the table in Visio, on the left hand side of the entity diagram, I see symbols next to the column names that I do not know what they mean. Symbols such as, U1, U2, l1, l2, l5...etc. Others symbols such as PK (primary key) and FK (foreign key) are also shown in the same area, but those obviously make sense to me. I am reverse engineering an database to create