When you include unfinished exercises in a portfolio, the simplest safeguard is to label each piece with a short context line that states what it is. A line like "personal type study, two hours" or "self-set brief exploring grid systems" tells the viewer exactly what they are looking at. This removes ambiguity without requiring a separate section or a disclaimer. The key is that the label appears at the same level of prominence as the work itself, not buried in a footnote or hidden behind a click.
Another useful approach is to describe the constraint you gave yourself rather than implying a client provided one. Instead of writing "logo for a coffee brand," which reads like a delivered commission, you can write "exploring how a single letterform can carry a brand voice" or "testing three color directions for a fictional brief." This keeps the focus on the skill you were practicing and makes it clear the project originated with you.
If your portfolio mixes finished client work and exercises, consider separating them by section or by visual weight. Giving client projects full case studies with outcomes and giving exercises a lighter treatment—fewer images, shorter text, a different background color or border—creates an immediate visual cue. Viewers learn the distinction within a few seconds of scrolling, and you avoid the awkwardness of someone asking whether a piece was real work.
Finally, resist the urge to polish an exercise until it looks indistinguishable from a delivered project. Part of what makes an exercise credible as an exercise is that it shows its own scope honestly. A rough edge, a visible iteration, or a note about what you would explore next all signal that this is a learning artifact. The practical benefit is that the piece stays consistent with what you can actually say about it if a viewer asks where it came from, so you are never in a position of having to explain away a gap between how the work looks and what it actually is.