Quick answer
An open bearing leaves the rolling elements exposed. A shield provides a non-contact metal barrier, while a contact seal generally provides stronger exclusion at the cost of additional friction. Design names and suffixes vary by manufacturer; confirm the exact construction rather than assuming all ZZ or 2RS products are identical.
Start with contamination
Identify dust, moisture, washdown, lubricant loss and maintenance access. Protection should fit the environment, rather than being selected from a suffix alone.
Consider friction and speed
Seal contact, lubricant and operating temperature influence friction and permissible speed. Use values for the complete designation and lubrication condition.
Specify what must be supplied
State the required closure on each side, lubricant and operating conditions in the RFQ. A dimensional match does not confirm an equivalent sealing design.