A placeholder is a specific value that is resolved at runtime against a property available in the property file set at component level. It is mainly used in the service unit JBI descriptor to be able to configure your service providers and/or your service consumers.

* if no property with name '{{placeholder-name}}' exists in the component property file, the default value '{{default-value}}' is used. If no default value is defined, the literal value '{{$\{placeholder-name}\}}' is used,
* if a placeholder name must contain the character ':' (colon), it must be escaped by the character '\', example: {{$\{placeholder-name-with-\:-colon:default-value}\}}',
* if a placeholder default value must contain the character ':' (colon), it must be escaped by the character '\', example: {{$\{placeholder-name:default-value-with-\:-colon}\}}'.

It is possible to change a placeholder value at runtime reloading the component property file. It is not sufficient, the parameter associated to the placeholder must be changeable at runtime. So check component documentation to know that.