What is the difference between using the "Client" versus "non-Client" versions of AWS API calls?

I am confused as to when to use the "Client" versus "non-Client" versions from the AWS SDK. In the code I reproduced below, I am creating a lambda function to send input text to Amazon Lex for processing. Can someone explain what the differences are and when to use the different versions?

import { LexRuntimeV2Client, RecognizeTextCommand } from "@aws-sdk/client-lex-runtime-v2";
import { LexRuntimeV2 } from "@aws-sdk/client-lex-runtime-v2";

export const handler = async(event) => {

    const LEXRUNTIMEClient = new LexRuntimeV2Client({region: 'us-east-1'});
    const LEXRUNTIME = new LexRuntimeV2({region: 'us-east-1'});
};