Sectional Lock vs Composite Timer, Explained
Roughly nine of the exams on this site lock you into each section with its own individual clock, while about twelve hand you one combined timer for the entire paper and let you move between sections however you like. Mixing up which one your exam uses (and practicing for the wrong one) is a common, avoidable mistake.
What sectional lock actually means
Under sectional lock, each section gets a fixed window and once it expires you're moved on automatically, whether you've finished or not: unused time in one section never carries over to the next. SBI Clerk Prelims gives you exactly 20 minutes per section this way, and RBI Grade B Phase I splits its four sections into separately timed 25/25/25/45-minute windows. If you're strong in one section and weak in another, sectional lock means you can't borrow time to compensate.
What a composite timer means
A composite timer gives you one clock for every section combined, and you decide how to split it. NABARD Grade A Phase I runs a single 120-minute timer across all eight of its sections, and SIDBI Grade A & B Phase I does the same across seven: you can answer every question in your strongest section first, then spend whatever's left on the rest.
Why this changes how you should practice
Practicing sectional-lock exams under an untimed or freely-navigable mock teaches you the wrong instinct: you need to feel the sudden cutoff, not just the total time pressure. Composite-timer exams reward the opposite skill: deciding, in real time, how many minutes a section is actually worth to you. Take sectional tests for the first kind and full mocks for the second, both are available on every exam's mock-test page on this site.
A third pattern worth knowing
SSC MTS & Havaldar does neither: it splits its CBT into two mandatory 45-minute sessions on the same day, with different negative-marking rules in each session. Always check the specific exam's pattern page rather than assuming; even within one recruiting body, the rules can change from one exam to the next (SSC CGL Tier 1 itself uses 15-minute-per-subject sectional locking, unlike SSC CHSL's single 60-minute composite timer).
One habit that works for both
Whichever pattern your exam uses, reviewing where your time actually went afterward matters more than guessing beforehand. See how to analyze your mock test performance for what to look at once you've got a result to study, and keep negative marking in mind: it changes the guessing math slightly differently under a hard section cutoff than under a flexible composite clock.