Lock - The terms refers to making a file unavailable to the other people for change. 

This as a team is very widely used in the document management systems. 

In concurrent version control softwares - Lock is offered as additional command as against check-out. 

In conventional version control systems - the check-out also makes the file unavailable to the other users for editing and modifications. 

In Concurrent Version Control Softwares - the users have to perform additional operation of lock. For authoring of documents that are primarily based on text - concurrent version systems allow the users to simultaneously and collaboratively work on the same files. 

For the users who work on vector format documents - engineering drawings and documents - where the visualization of the differences between the versions is difficult, it a normal practice to lock the documents after the checkout.   

tBits CollabWrite offers the concurrent version control features as in a project there are as many documents that are text based - Calculations, Design Basis Reports, Monthly Progress Reports, Specifications, Tender Documents, Technical Evaluation Reports etc as there are number of drawings. The users - at the time of check-out - can also lock the documents - which is widely used at the time of modification in engineering drawings.