Does purchasing REs guarantee that a share will be given or is it like subscribing to an IPO?

Purchasing Rights Entitlements (REs) gives you the right, but not the obligation, to buy shares in a company's rights issue at a discounted price; it's not a guarantee of allotment, similar to an IPO where allotment isn't guaranteed if oversubscribed. You must apply for the shares using the REs you own, and if you don't, or if the issue is oversubscribed, you will not receive the rights shares, potentially losing the amount you paid for the REs.