What does mean VSL is similar to VSU?

VSU and VSL is different.
a) For VSU feature
VSU is a technology that virtualizes multiple devices into one device for management and use, which can significantly reduce management complexity and topology logic complexity. Compared with MSTP+VRRP network, VSU usually uses aggregate ports to connect with peripheral devices. , which can effectively utilize redundant links and improve the forwarding capability of the system.
b) For VSL
A Virtual Switching Link (VSL for short) is a special aggregated link that transmits control information and data streams between devices in a VSU system. A VSL port exists in the form of an aggregated port group, and the data flow transmitted by the VSL is load balanced among the members of the aggregated port according to the traffic balancing algorithm.
The physical ports on the device that can be used for VSL port connections are called VSL member ports. VSL member ports may be stack ports, Ethernet ports, or optical ports (which ports on the device can be used as VSL member ports are related to the model of the device, please refer to the actual situation of the device).
c) Typical topology
Here are two types of topology for VSU and VSL

Linear topology

Ring topology

You can refer to this post to configure VSU on Ruijie switches

How to configure VSU?