Apple has rejected the app and suggests two options : Volume purchase program and Enterprise program I have develop the app for a client who has its own company, the employee within the company or any external user can use this application. 
But first user need to register himself to use this app.
While registering user needs to give the unique code. This will be provided by company, user need to contact first to company. 
This is the my app's registering senario.

I have submit my app to apple for review with proper metadata and all apps senario.
 But i dont know how apple has considered it as inhouse app, which have the ability to make it use for external user.
But apple reject it and suggest to distribute it with volume purchase program and enterprise program.

After lots of searching I understood following things about these: 

VPP(volume purchase program): We can purchase the application from vpp store in a bulk and then the company owner
 who has the vpp account can distribute amongst its employees.
Enterprise Program : In this program we need to upload .ipa file to our custom server and make it available to employees by providing downlod link.

This is all my understanding about these two concept.
One confusion about vpp that, Are vpp only contains the app listed that are available in app store?(if yes then my app is not goig to be distributed in app store(according to apple) then how apple suggest vpp)
Can any one please help me out? and please suggest how to make it approved from apple to distribute in app store? Any help is appreciated.
Thanks in advance..
If your app is in fact targeting both employees and people external to the company, then the Public App Store is your only choice anyway. All other options like VPP or the Apple Developer Enterprise Program do only allow you to distribute apps to employees. So your argumentation towards Apple would be that it needs to be the public store, because there is no other way to distribute that app to a potentially global audience.

However they might not like your authentication method "unique code", as Apple usually wants apps to have a signup method and a more sophisticated login, such as username and password.

Adding some external (publicly visible) content near the login might help to get the App approved as well.