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By default, the Gantt chart only shows utilization within the scope of the chart – so any work planned for issues that aren't in your structure (and Gantt chart) won't be accounted for.
Using Planner data, Structure. Gantt is able to consider allocations from any issue, even if it isn't in the chart, when considering resource allocation. So if you're focused on scheduling Project A, but someone on your team already has work planned for Project B, that time will be considered when determining their availability.
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