Deciding what strategy to use is purely up to you. You can use the short description in the SD-WAN Rule configuration page to decide, or you can look at Fortinet's documentation for more examples and details.
Best Quality is mainly based on the measured SLA and selecting the criteria that is most important to you.
Lowest Cost is a manual setting. You can input under SD-WAN Member config what each interface "cost" is. This could be the actual cost of the line, a cost based on price for over usage, or anything you deemed to factor into cost.
Maximum bandwidth is actually a load-balancing scheme among available links that meets SLA
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