A redundancy pool is a group of employees from which redundant staff will be selected. A redundancy pool is not relevant in smaller businesses, where only one employee performing a unique role is being made redundant.
Each pool typically includes employees who currently perform the same or similar roles or will do so after the redundancy exercise, if not made redundant. So, you may need a few different redundancy pools for different business divisions or areas (eg marketing, sales, IT).
Redundancy pooling is not usually relevant where there is no need to select between employees performing a similar role (ie where a particular role will no longer exist, with no replacement or an entire workplace will shut). This will mostly be the case for small or micro businesses, where employees perform their role individually.
Choosing the right redundancy pools has a subjective element and there is no single right approach but the employer must act reasonably and take care to avoid illegal discrimination.