Current transformers are rated at both primary and secondary windings. In a standard setup, the primary winding consists of the conductor passing through the transformer, while the secondary winding has the coil around the core hooked to a metering device like an ammeter. Consequently, a transformer with a 100/5 spec means the primary current is at 100 A, while the secondary current is at 5 A.
The use of the term “windings” is universal for both current transformers and transformers in general. However, current transformers are more concerned with measurement than stepping voltages up or down (although they have this capability, to some extent). The primary current is current from a power source, while the secondary current delivers the power to the load (i.e. ammeter).
Normally, you can’t see the secondary winding because the transformer encases it in its shell. A cross-section of a solid or split-core transformer, however, reveals three important parts: the magnetic core, an insulated capsule enclosing the core, and windings wrapping the capsule. The magnetic core enables the transformer to perform its task via the Hall Effect.
In the current transformer market, the secondary current is almost always 5 A, while the primary current varies greatly.