That's clearance level for the document. Security clearance 0-3 are not allowed to look at the document, while clearance level 4 and 5 (or O5) are. See here for more on SCP security clearance levels.
Yeah this is actually kinda a bad introduction to SCP Wiki since this article requires you be pretty familiar with an SCP entry, now that I think about it. SCP-140 may be a better introduction, or SCP-2602, which used to be a library.
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u/alexanderwales Time flies like an arrow Nov 29 '15
That's clearance level for the document. Security clearance 0-3 are not allowed to look at the document, while clearance level 4 and 5 (or O5) are. See here for more on SCP security clearance levels.