Editing Preferred Roaming List (section)

==PRL modes of operation==
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The PRL may operate in one of two modes, dictated by a Preferred Only indicator:
:*'''Permissive Mode''' (''a.k.a. Open PRL, Not Preferred Mode'') – The device will attempt to acquire preferred systems; however, if no preferred systems can be located, then the device may select systems that are not listed in the PRL system table. It still will not select negative systems; but, if no preferred systems can be found and an unknown system is available, then it will select the unknown system. While this mode of operation is easier to maintain, because new partners can be chosen even if they are not explicitly listed in the PRL, it carries with it the danger of allowing the mobile device to become camped onto a system that will not provide it with service.
:*'''Restrictive Mode''' (''a.k.a. Closed PRL, Preferred Only Mode'') – The device will only acquire systems identified as preferred in the PRL system table. While this mode of operation gives operators the most control over the system selection process, it requires that PRLs be kept up-to-date to ensure that newly added roaming partners can be selected. Note that even if a PRL is restrictive according to the preferred only indicator, the use of wildcard SID/NID or MCC/MNC values as preferred systems would essentially make the PRL permissive, because wildcards match any system that is located.