A client portal is functionally identical to a customer portal — the difference is mostly framing. “Client portal” is more common in professional services (legal, accounting, consulting, agencies), where the relationship is project-based and high-touch. “Customer portal” is more common in SaaS, e-commerce, and B2B, where relationships are typically more transactional or subscription-based.
The underlying technology and features are the same: secure login, document sharing, messaging, billing, project tracking. The vocabulary you use depends on what your audience expects.
For a deeper comparison, see Client Portal vs. Customer Portal.