What it does
Models BIG-IP DNS (formerly GTM) global server load balancing, which is two-tier: a wide IP first selects a pool, then the pool selects a virtual server. Configure your pools - each with a ratio, a region tag, an up/down state, and a member-selection method - and their members, then set the wide-IP pool method, a client region, and a request count N. The tool computes how the next N DNS name-resolution requests resolve: first the per-pool distribution, then the per-member distribution within each pool.
What it simulates, and what it does not
Deterministic at both tiers: Round Robin (circular and even), Ratio (weighted, largest-remainder), Global Availability (all requests to the first available resource in list order, the rest as backups), and Topology (the resource whose region matches the client wins with the highest score; ties inside the winning score round-robin). A down pool or member receives nothing, and the traffic redistributes among those that are up.
The dynamic methods - Quality of Service, Completion Rate, Round-Trip Time, Fewest Hops, Kilobytes/Second, Packet Rate, Virtual Server Score, Least Connections, CPU - depend on live performance metrics that the big3d agents collect from each data center. Those cannot be reproduced offline, so the tool says so plainly rather than inventing a distribution. This is the same honesty rule the LTM simulator follows for its dynamic methods.
Why two tiers matter
The most common GSLB confusion is treating pool selection and member selection as one decision. They are separate, and each can use a different method: a wide IP might pick pools by Topology (send European clients to the EU pool) while each pool picks a virtual server by Ratio (weight the bigger site heavier). The tool makes that separation visible - you see which pool answered, then which virtual server inside it resolved - so the interaction between the two tiers is concrete rather than abstract.