SAP's Concurrent Employment (CE) functionality has been out for a few years now, and several customers have successfully implemented it. This CE functionality enables you to have multiple 'assignments' per person in the HR system. Think of an assignment as a job or position (not necessarily 'job' or 'position' in strict SAP-terms, but in a business-sense). If you have a sizable population of employees who work in multiple jobs or positions, then CE might be right for you.
In the traditional SAP HR model, an employee has one job or position at a time, keyed by a Personnel Number (i.e. 'pernr'). This works fine for most employers, but not those where a given employee may hold two or more jobs at the same time. For example, in a university a management staff person can also be a part-time faculty instructor. In a hospital, a doctor or nurse may work in multiple tax entities within the same corporation. Each of these cases requires the person to hold multiple positions, rates of pay, perhaps benefits plans, and other work-related information such as schedules and tax authorities.
It is possible to have one 'pernr' and be paid via multiple wagetypes charged to various funding sources. The person is still assigned to one position in their Organization Assignment, and their pay is taxed according to the same set of work tax authorities and in the same Federal and State employers, for example. If you can meet your business requirements using multiple wagetypes and various funding sources, then you don't really have a need for SAP's CE functionality.
However, if that person really needs to be a holder in each position – Management Staff and Part-time Faculty - then that's a good indicator you need CE. If that one person actually works in two different employers, splitting their reportable wage and taxes, then that is another good case for CE. Those are two of the primary drivers of CE business cases, though some companies might have additional cases that warrant CE.
Now, once you decide you need CE functionality, employees have more than one Personnel Number, or 'pernr'. In the CE environment, this is now referred to as their Personnel Assignment, though the actual field name is still 'pernr'. Linking those multiple assignments together is a Person ID – the one unique identifier for the person. So in CE, Person ID is the new Personnel Number.
There are many other details about CE that you need to consider – such as all assignments belonging to the same payroll area and having a new payroll reporting structure – so this is just a starting point. The CE functionality isn't so new any longer, and it is much more stable (so you can start discounting the usual early-adopter horror stories), and it deserves serious consideration for every company that has a significant population of workers with multiple jobs.