i have two custom fields in suitelet, field A and field B, if field A chose with one customer identify its subcustomer's and display it as list in field B as options. once value populated list of subcustomer in field B , if user choose one customer from Field B, i need to set field A with the value choose in Field B. how to achieve it?

client script. suitelets don't have nice sourcing and filtering options

you can upload a clientscript and load in suitelet, BUT, in 19.2 you need the FILE ID in file cabinet. You can find using a search file module, I use in some suitelets in Avalara and works very well.

if you want to attach a client script in a suitlet, i recommend Form.clientScriptModulePath over Form.clientScriptFileId

I am not worried about attaching ClientScript