An ice pack straight out of a freezer can be well below 0 °C. Placed beside a vial of a freeze-sensitive vaccine in a carrier, it can push the vaccine below 0 °C and damage it. Conditioning solves this: the pack is taken out of the freezer and left at room temperature until the ice inside starts to melt and the pack's surface is at about 0 °C. A commonly taught check is that water can be heard moving when the pack is shaken.
Conditioned packs are used for carrying vaccines that must stay within +2 to +8 °C. Some programmes use fully frozen packs for specific vaccines that tolerate freezing. The exact method, and which vaccines go with which packs, is set by the immunisation programme, and health workers should follow those instructions.
At a community health centre in Bihar, the staff might take ice packs from the deep freezer an hour before outreach sessions, lay them out on a table to condition, check each one by shaking it, and only then pack them into vaccine carriers.