When a manuscript's status switches from "Under Review" to "Ready for Decision," it implies that the peer-review process has been completed and just the editorial decision is pending. It is preferable to contact the editor to find out where the work stands.
This might take anything from 2 to 6 weeks at times. Longer periods are unusual but not unheard of.
The editor will notify you by email when your manuscript has been accepted or rejected. If it has been rejected, you will be given reason(s) why it was rejected and what changes need to be made before your manuscript can be considered again. Typically these changes are minimal and can be made quickly in order to resubmit a new version of your manuscript.
It depends on the journal and how fast they like to make decisions. Some journals may accept only one version of a paper so if you have major revisions you want to make they will have to be made within a few months of initial submission. Others allow authors to revise their papers repeatedly until they are happy with them.
In general though, you should receive an answer within 4-6 weeks of submitting your paper. If you do not hear from anyone for several months then assume the paper was rejected.
When your paper's status changes from "Under Review" to "Pending Recommendation," it implies that peer review has been completed, and the Associate Editor (AE) will now read through the reviews and make a recommendation on whether it should be accepted, rejected, or changed.
The AE can accept any or all of the comments made by the reviewers during this process. He or she may also add additional comments of his own. Then, based on these recommendations, the Editorial Manager will update the paper's status again, this time to "Accepted." The papers that are not recommended for publication but which the authors feel must be published in order for their work to be complete will be sent out for further consideration by other editors. This is called "pending revision." As with papers in "under review" status, AEs can recommend that these papers be rejected, with or without revisions.
Finally, there are two other statuses that can be applied to a paper: "Review Draft" and "Revised Manuscript With Comments." These papers have gone through initial peer review but not yet final editorial review. They are still being worked on by the editor who first received them. These papers are considered ready for submission to another journal. Once they reach this stage, they cannot be submitted to any other journals until they are accepted or rejected by this first editor.
"Decision in progress" indicates that all of the reviews have been received and that all that remains is for the editor to make a decision. Three weeks is a long time for this. If you want something done fast, don't use this channel.
The phrase "decision pending" indicates that the decision is still pending. They haven't made a decision yet. Maybe they will, maybe they won't. The fact is that they can't make a decision until all the evidence is in.
There is no hidden code that can be used to decipher the submission tracker. Things often take considerably longer than we anticipate. The best thing to do if you don't hear back from them is to assume that you weren't selected for this position.
In response to your question, "Decision in Process" normally implies that the associate editor (AE) has reviewed the peer review comments, made a decision on the manuscript based on the comments, and then informed the Editor-in-Chief of their decision (EiC). The AE may consult with other members of the editorial board as needed.
However, "decision" can also imply that the EiC has made the final call on the manuscript's acceptance or rejection. For example, if the EiC rejects a manuscript immediately after reviewing it, they may not need to seek additional input from AEs before making their decision. However, if the EiC wants more feedback from other members of the editorial board, they would send the manuscript to one or more of them for review before making their final decision.
As another example, if the AE feels that some major issues need to be resolved before accepting the manuscript, they might ask the EiC for more time to work on it first. Once these issues are resolved to their satisfaction, they would send the revised version of the manuscript to the EiC for final approval before going into production.
Finally, even if the AE tells the EiC that they want further work done on the manuscript before making a decision, this does not necessarily mean that the decision is pending until then.