Hi @signils ,

I think it would be ideal if you can use IFS Help documents as it contains detailed information regarding these features. But In summary;

• Customer Invoices without customer order is referred to Manual Customer Invoices and Instant Invoices which can be used to create invoices without having a Customer Order such as Services, Charges or Non-Stock Products. 
• Customer Credit Management is used maintain Customer Credit Information which used to handle assignment of Credit Analysts to Customer for tracking and monitoring purpose. Various Credit Related reports such as  DSO- Days sales outstanding and Follow-up reminders can be generated if you are using credit management functionality.
• Cash Book is used to maintain all the Payment related information (Mixed Payment, Cash Box, Bank Reconciliation)and basic data for payment related documents. All the Payment information done in IFS Application are comes under Cash Book.

Hope this will help.

Regards,

Akila