Given the following file in a shared library's vars folder

class acme implements Serializable {
    
    def script
    
    acme(script) {this.script = script}
}

And the following snippet of Declarative code

stages {
        stage("Checking component sources and building snapshot") {
            steps {
                script {
                    acme = new acme(this)
                }
             }
           }
}

The end result is an error like

Unable to resolve class acme

To make this work I need to move the acme initialization out of the pipeline block like this

def acme = new acme(this)

pipeline {
 .......

Although that is good to me as now the acme instance is available to all stages the current documentation does not mention anything related to this and I believe is worthwhile to add this case to the documentation.