Form Select
Default Select
Use <select> attribute with numerous options to show value with choice's option.
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Menu Size
Use multiple attribute to select multiple values and size="your value" attribute to show a number of choice option within a select area. 
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Select Size
Use form-select-sm class to set small select size and form-select-lg class to form-select class to set large select size. No such class is required for default select size.
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Choices
Single select input Example
Set data-choices attribute to set a default single select.
Set data-choices data-choices-groups attribute to set option group
Set data-choices data-choices-search-false 
Set data-choices data-choices-sorting-false attribute.
Multiple select input
Set data-choices multiple attribute.
Set  multiple attribute.
Set data-choices data-choices-multiple-groups="true" multiple attribute. 
Text inputs
Set data-choices data-choices-limit="Required Limit"  attribute.
Set data-choices data-choices-text-unique-true attribute.
Set data-choices data-choices-text-disabled-true attribute.