Acclimation helps flooring products adjust to the temperature and humidity on the job, which can prevent problems with expansion and contraction of materials as they are exposed to changing temperatures. Acclimation is a common practice in the wood and laminate industry, but is often ignored on the resilient or carpet side. For example, installing resilient material that is warm can lead to shrinkage problems later on, and cold materials are harder to work with and may tend to expand when they get warm.
In warm temperatures, care needs to be taken because it's not that the material actually shrinks after installation, but that it stretches during handling if it is warm, or may "grow" slightly.
Rectangular products such as wall base, edgings or vinyl plank are of particular concern. These products are easy to stretch during handling by carrying cartons over your shoulder and allowing the box to bend, pulling warm material out of the end of the carton, or stretching the material end to end as you are setting it into the adhesive. Whether the installer stretches it or it has expanded slightly, the material installed in this state will look good with nice tight seams until the temperature cools and the material returns to its original size. The gaps that show up when this happens are thought of as "shrinkage," but the material is in fact returning to its original size. When you notice an installation of vinyl flooring where the floor tile, reducers, or the wall base is gapped, it's a good bet the job was done in the summer time and the material was not acclimated.
All flooring materials have acclimation recommendations as part of the installation instructions, but are often ignored. Following them avoids some of the problems mentioned here, thereby reducing callbacks and eliminating complaints.