A system for use in a subsea well application, comprising: a subsea tree having a tree cap receiving area in communication with a production passage and with an annulus passage via a crossover port in the subsea tree; and a tree cap sized for receipt by the tree cap receiving area for removable engagement with the subsea tree, the tree cap comprising: a locking feature selectively actuated to lock the tree cap to the subsea tree; an ROV operated mechanism interacting with the locking feature to enable locking and unlocking of the tree cap via a remotely operated vehicle; an upper sealing mandrel connected to the ROV operated mechanism; a lower sealing mandrel connected to the upper sealing mandrel; and a seal system positioned to seal to the subsea tree and provide a desired flow isolation with respect to the production passage and the annulus passage when the tree cap is engaged and locked with respect to the subsea tree, wherein the seal system comprises an upper seal on the upper sealing mandrel and a lower seal on the lower sealing mandrel, the upper seal being located above the crossover port and the lower seal being located below the crossover port when the tree cap is fully inserted into the tree cap receiving area; wherein the ROV operated mechanism comprises an ROV interface configured for the mechanism to be rotatable by application of torque thereto by a tool of the ROV and rotation of the ROV operated mechanism actuates the locking feature to lock the tree cap to the subsea tree. a subsea tree having a tree cap receiving area in communication with a production passage and with an annulus passage via a crossover port in the subsea tree; and a tree cap sized for receipt by the tree cap receiving area for removable engagement with the subsea tree, the tree cap comprising: a locking feature selectively actuated to lock the tree cap to the subsea tree; an ROV operated mechanism interacting with the locking feature to enable locking and unlocking of the tree cap via a remotely operated vehicle; an upper sealing mandrel connected to the ROV operated mechanism; a lower sealing mandrel connected to the upper sealing mandrel; and a seal system positioned to seal to the subsea tree and provide a desired flow isolation with respect to the production passage and the annulus passage when the tree cap is engaged and locked with respect to the subsea tree, wherein the seal system comprises an upper seal on the upper sealing mandrel and a lower seal on the lower sealing mandrel, the upper seal being located above the crossover port and the lower seal being located below the crossover port when the tree cap is fully inserted into the tree cap receiving area; a locking feature selectively actuated to lock the tree cap to the subsea tree; an ROV operated mechanism interacting with the locking feature to enable locking and unlocking of the tree cap via a remotely operated vehicle; an upper sealing mandrel connected to the ROV operated mechanism; a lower sealing mandrel connected to the upper sealing mandrel; and a seal system positioned to seal to the subsea tree and provide a desired flow isolation with respect to the production passage and the annulus passage when the tree cap is engaged and locked with respect to the subsea tree, wherein the seal system comprises an upper seal on the upper sealing mandrel and a lower seal on the lower sealing mandrel, the upper seal being located above the crossover port and the lower seal being located below the crossover port when the tree cap is fully inserted into the tree cap receiving area;