Omni-Channel is a work routing system.
Omni-Channel works on the concept of Work Items. Each time a case is assigned to a queue, a work item is created and pushed to the agents. Omni-Channel tracks the number of work items assigned to an agent based on number of primary tabs related to the work items are open.
Set how much work your agents can handle at once. Size your chats relative to agent capacity so that agents can handle multiple chats, but aren’t overworked.
In Work Item Size, set the amount of an agent’s capacity that one work item consumes.
In Agent Capacity, set the agent’s total capacity.
Units of Capacity indicates the amount of an agent’s overall capacity that’s consumed when the agent is assigned a work item from queues that are associated with this configuration.

The Capacity setting in the presence configuration the agent is assigned to determines the agent’s overall capacity. When the agent is assigned a work item from the queue that’s associated with this configuration, the Capacity Weight is subtracted from the agent’s overall capacity. Agents can be assigned work items until their overall capacity reaches 0.

You can select a Capacity Weight or a Capacity Percentage, but not both.

https://help.salesforce.com/articleView?id=000338278&type=1&mode=1