What's the difference between these two marking types?
The part markings are the text stamped or engraved on the physical ICs.  There are two different part markings here because the LTC3526L and LTC3526LB are two functionally different parts.
The difference between those parts appears to be in how they handle light loads.  From the bullet points on the first page of the datasheet:

And from further along in the datasheet, under the heading &quot;Burst Mode Operation&quot;:

The LTC3526L will enter Burst Mode operation at light load
current and return to fixed frequency PWM mode when the
loadincreases.

and

The LTC3526LB features continuous PWM operation at
1MHz. At very light loads, the LTC3526LB will exhibit
pulse-skip operation.

Note that in most places the datasheet refers to &quot;The LTC3526L/LTC3526LB&quot;, whereas in these places it refers to only &quot;The LTC3526L&quot; or &quot;The LTC3526LB&quot;.  There may be other differences, but on a cursory reading of the datasheet this appears to be the most important difference if not the only one.