What are the Omni-Channel routing models and when do you use each?

Omni-Channel supports three routing models:

1. Most Available — routes work to the agent with the highest ratio of open capacity to total capacity. Best for even workload distribution.
2. Least Active — routes to the agent with the fewest open work items, regardless of capacity percentages. Best for equal work-item count distribution.
3. External Routing — integrates with a third-party routing engine via the Omni-Channel API; Salesforce passes work item details to the external system, which determines assignment. Additionally, Skills-Based Routing (an enhancement) matches work items to agents based on required skills defined on the queue or routing configuration rather than just availability.