Three systems. Three different games.
Almost every counselling guide gives the same "fill your preferences carefully" advice for all three exams. The mechanics genuinely differ — and the costly mistakes are exam-specific.
JoSAA
Your preference list locks once submitted. Float is close to a free option — it can never cost the seat you already hold.
MCC (AIQ)
Round 1 upgrade eligibility requires physically joining — documents, fees, travel. The portal click is the smallest part.
DU-CSAS
The one system that gives you a second chance: a vacancy-display window where you can re-order preferences mid-process.
v1 scope: JoSAA (all-India), MCC AIQ (all-India quota), and DU-CSAS as the CUET reference flow. NEET state quotas and non-DU CUET universities run their own separate processes with their own rules — coverage for those is a planned expansion, and we'd rather say so than pretend.