A voluntary bid of 5NT is forcing and asks partner to try to pick the best slam available. There are no hard-and-fast rules here, but possibilities include:
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bidding a new 4-card suit (looking for a 4-4 fit)
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showing secondary support (looking for a good 7-card fit)
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rebidding a previously bid suit with unannounced length and/or strength
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returning to 6NT with no better options
The asker may continue to bid up the line if completely displeased with the response.