I am no expert here, and anyone please feel free to chime in and elaborate/correct where needed.
Cognitively, ISFP's and INFP's share Fi and Te, but differ in their 2 middle attributes.
INFP = Fi Ne Si Te
ISFP = Fi Se Ni Te
Ne - Interested with other perspectives, applying vast insight to see all sides of an object, to understand the object completely.
Se - Interested in the colors, lines, texture, taste, smell of an object. Observing the setting of the object.
Si - Recalling past objects that might relate, facts about similar objects. Memories that link to the object.
Ni - Wondering why the object is there, what is its purpose? Does this object have any potential impact on the future?
Someone asked for a practical example of difference between the two.
So in this scenario a boy is helping his father build a playset. The boy is in charge of providing the tools needed. This contrasts the boy being ISFP vs INFP.
In many cases the ISFP boy has a good idea of where everything is at due to Se but he may become "mesmerized" just studying the objects, not necessarily thinking about them, just looking at the hammer, picking it up and feeling the weight, noticing how cold the metal is, etc. The Ni may cause the ISFP boy wonder why we have playsets, what is the purpose? If I didn't have a playset, would it change me?
The INFP boy might not pay as much attention to how things are laid out and instead get lost in a daydream about who knows what? Some may think about the wood used to create the playset, where did it come from, what trees died? Where did the metal from the screws come from? What would the Native Americans think if they stumbled upon a playset like this before we took their land? Si may compare this playset to his friend's playset, the differences. Then Si may pull the INFP into a recall of fond memories playing at his friend's house.
In both cases, the boy may try to organize the items to make the process more efficient (Te). If the father gets angry at the boy, the boy will be intensely hurt, but most likely will not show it. (Fi)