It means they've reserved the right to do an offering of securities up to the shelf registration limits at any time as needed (i.e. pull it off the shelf). In this case they have not set any limits. However, they are required to issue a registration supplement beforehand which specifies the details/limits if and when they actually offer/sell any new securities.
Basically, the shelf registration makes the process a little shorter and the paperwork/bureaucracy a little less burdensome if/when they need to draw on/issue additional shares to raise funds. It also puts their shareholders on notice that they may issue new shares in the future.
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