What special waqf issue does the source discuss for «سَلَاسِلَا»?

Rules of Waqf — Correct Stopping, Resumption, Rawm, Ishmām and Qurʾānic Orthography

• It always has a doubled nūn
• Both keeping and omitting its alif are reported
• Saktah is mandatory in the word
• It ends with tāʾ marbūṭah