Section classification (compact/non-compact/slender or Class 1–4): why it changes which formula applies
A steel section is made of thin flat plates welded or rolled together — and those individual plates can buckle locally, on their own, before the section as a whole ever reaches its full theoretical capacity. Classification is how codes account for that.
A rolled or fabricated steel section — an I-beam's flanges and web, for instance — is really a set of individual flat plate elements joined together. Each of those plate elements can, if it's thin relative to how wide it is, buckle locally on its own under compressive stress, wrinkling or waving in a way that's completely separate from the overall member buckling (or lateral-torsional buckling) of the section as a whole. A section built from thin, wide plates can start losing capacity to this kind of local buckling well before the material itself would otherwise reach yield — which means the section's theoretical full capacity, calculated by assuming the material simply yields everywhere, may not actually be achievable in practice.
Section classification is exactly how codes handle this: sorting a given section into a category based on the width-to-thickness ratios of its individual plate elements, describing how much of that section's theoretical capacity can genuinely be relied on before local buckling interferes.
Two labelling systems, the same underlying idea
EC3 uses four numbered classes: Class 1 sections can reach full plastic capacity and sustain the rotation needed for plastic analysis of the whole structure; Class 2 can reach full plastic capacity but without that extra rotation capacity; Class 3 can only reach yield at the extreme fibre before local buckling limits it, so it's designed elastically rather than plastically; Class 4 experiences local buckling before yield is even reached anywhere in the section, and has to be designed using a reduced "effective" section that discounts the part of the plate that can't be trusted. AISC 360 uses three descriptive categories instead — compact, non-compact, and slender — covering broadly the same underlying idea in a different structure. The specific width-to-thickness boundary ratios that decide which category a given plate element falls into genuinely differ between the two codes, and are exactly the kind of number this entry deliberately isn't going to reproduce — check EC3 or AISC 360 directly, or let a calculator apply the actual code check.
The practical consequence is real, not just a label: a compact or Class 1/2 section can be designed assuming the whole section yields (plastic design), which is more capacity-efficient, while a slender or Class 4 section has to fall back to a more conservative effective-section approach. Getting the classification right changes which formula legitimately applies — treating a slender section as if it were compact would overstate its real capacity. Steel Beam Design classifies the section you're working with directly as part of its own design check, and Section Properties is where the geometric ratios that classification is built from come from in the first place (see the related tools below).